I use Quickbooks for both businesses. I find it very easy to understand, easy to teach, and easy to send to the accountant.
I am familiar with Peachtree but I do not find it nearly as intuitive as Quickbooks. I have used Quickbooks Customer Manager which works as a stand-alone program or in conjunction with Quickbooks. It adds the ability to keep more detailed information on a client. It does not do any billing or anything like that.
I've found with the latest Quickbooks Pro 2006 that I no longer need the Customer Manager solution as the Quickbooks software itself has added the features that for which I used Customer Manager. As a beta tester for Intuit I was really impressed with the 2006 version. It had really good changes to make it even easier that ever.
For a small business with no inventory you may want to consider the SimpleStart version. I think that would do everything you would need. The suggested retail is around $99 I think, but I've seen it for as low as $75.
You may want to bend to the accountant on this one if the accountant has a preference. When I need to do my taxes or anything I just save a file on to my thumb drive and take it to the accountant...not one shred of paper. Its great.