hi natalie,
if you're going for a new computer, then you'll want a pci-e card (and a pci-e compatible motherboard). that's pretty much where the industry is going. they are faster than agp and have all the new shader technology and so on built in. i'd also recommend getting one with at least 256 megs of onboard memory. photoshop and the files you load can eat memory in short order. so, the more is always better.
as for specific recommendations, i'm really not qualified these days. i've been out of the whole techie thing for a while now. you might try some place like 'Tom's Hardware'. they shld have plenty of reviews on hardware. you're going to run into terms like 'sli' and others. sli is basically two video cards linked together with another processor/controller. great for games, but you dont really need it for photoshopping. one pci-e card shld suffice. as for dual monitors, again, you'll have to check the specs on the individual cards.
you might also try sites like
http://www.resellerratings.com/ . these places compare prices and give some reviews and user feedback.
http://www.newegg.com might also be a good place to check prices. havent been there in a while but they are a reputable online store.