ah, the old hardware upgrade question. i'm a bit out of the loop right now, being that i havent upgraded in a while, but i'll throw some bolts in the works.
you actually have three things to consider here; hardware, software, and net connection since you mentioned downloading speeds.
HARDWARE:
you can never have too much ram, especially in image editing! make sure you check motherboard specs closely here. a motherboard might say something like 'up to 4 gigabytes of ram'. make sure you look closely at the type of ram and how they're going to configure it.
windows now has a 64 bit windows XP and it appears to work.
next to ram, cpu speed and vid card are of comparable importance in speed. a slow vid card can kill you no matter how fast your cpu is, and vice versa.
the pci-e (pci express) cards were originally used by alienware. they consist of TWO pci-e vid cards (when used correctly) and a third controller device to sync the two cards. you MUST have a compatible motherboard! one vid card controls the upper half of your monitor and the other vid card controls the lower half. the controller keeps them synced. thus, if one card were broken or turned off, you'd only see half of your display on the monitor. these are without question the FASTEST vid cards currently on the market.
if you're not using pci-e, then use agp 8x, 16x. you wont really get 16x, but make sure it allows for this.
with agp vid cards, i wouldnt go for anything less than a 9800 ati or the comparable nvidia card. the 9800 ati is a solid card. your 'all in wonder' card is a 9800 that has tv tuner capability and live capture capability. it's more pricey than a stock 9800 and most dont need it. but, it is handy for capturing live video off a tv, vcr, dvd, cd player and so on.
part of the reason for the pci-e cards being so fast is buss speeds on the motherboard. but this also applies to other hardware. one thing to look for in any motherboard is it's FSB speed, or front side buss speed. the faster, the better.
to get the full use and speed of a 64 bit motherboard, you'd preferably run 64 software. this is somewhat lacking still. most is still 32 bit. you
should see some slight increase regardless of whether you're running 64 bit or 32, however, but it may be very slight.
to run 64 bit you need a 64 bit processor. a 64 bit processor needs a 64 bit motherboard.
motherboards come with a lot of bells and whistles these days. a number of things to look for are read ahead buffering, auto-heat monitoring and monitors, auto-fix POST, usb, firewire, and audio ports on the front side of the computer, dual monitor capability, overall number of usb ports (lots of scanners use usb now and so do other devices. i wouldnt want anything less than 4, with 6 or 8 being preferred), and probably a host of other things i'm forgetting.
sata and ide support. the newer sata drives are faster than ide devices for the most part, but dont buy a motherboard without ide ports. you might also be interested in RAID. there are a number of RAID types. study the types and get what you really want. or, dont use them all.
generally speaking, if you can build your own computer, this is the way to go. you'll know EXACTLY what's in the box. OEM computers tend to cut corners and you may not always like what they cut. if you do go OEM, make sure you ask ALL the questions!
SOFTWARE:
whenever possible, buy the windows disk. OEM windows is sort of like a cul-de-sac and you can end up with nowhere to go but re-formatting.
you can have all the greatest hardware in the world and still run like a chihauhau at a greyhound race. the biggest culprit is usually windows itself, especially XP (pro or home). windows is a hog! plain and simple it just has too much junk going on. NOBODY needs it all. it's a hog. a stock installation of windows puts about 50 things running in the background whenever you boot up. most if it is not really necessary to most people. the first thing i do whenever i install a new windows o/s is to go to
www.blackviper.com and look at their trimming service. they list all those idiot processes and services that windows runs and tells you what you can just kill permanently and what to set to manual and what to leave on auto.
photoshop and paint shop pro are fairly large programs. i dont know anything about bridges. but software can kill hardware or at least make it so seem so slow that the hardware wishes it were dead

check out your software. turn off features you never use. look at user groups and see what they have to say about speeds and loading times and so on. check review pages on the net also.
for things like batch processing, check out dedicated programs for this sort of thing. Irfanview is probably one of the best and there is a free version. and i'm sure there are others.
for thumbnails follow the same general tips as the batch processing. look for something faster.
NETWORKING:
generally, you're dealing with two types of networks these days, the internet and LANS. since you didnt mention LANS, we'll skip that one. your downloading woes are not the fault of your computer, most likely, but rather your software or more likely your net connection. net speeds are dependent on several things and not just 'my modem sucks'

your basic top end is determined by your isp and what type of connection you've paid for. you can get anywhere from 56k to the new EXTREMELY fast connections running 10 mbits or better. you didnt mention what you have, so i'll save all this for later. i did mention that software can slow your net connection. an improper installation of isp net software can kill you. poorly configured net settings can slow you. active virus checking can slow things a great deal when dealing with file downloading.
'throughput' can also slow you to a crawl. when you connect to somewhere you are going through any number of 'hops'. these are nodes or terminals which route you hop by hop to your final destination. net traffic through those various terminals can slow you. just the number of hops you have to make can slow you. this isnt quite as bad as it used to be. routing technology has gotten better. but you can still get 'jammed up' going through some hubs and even smaller connections. and, the server on the other end may just have a very small pipeline. you may have a 5 mbit connection, but if the guy on the other end is running 56k, you're going to go at, guess what, 56k; at least when connecting to him.
ok. that's enough. i'm sure gary or someone else is more up to date on this and is going to correct about 50% of what i just said

so, i'll just close with do comparitive shopping. there is a real war for your business going on in the tech area. shop around and i'll bet that $1000 will get you a pretty good machine.
Craig