Pretty much any computer these days that retails for $500 and up will perform the basics of what you need subject to the recommendations below. My primary suggestion is to ensure that you have a minimum of 4GB of RAM on the machine when using a 32-bit OS such as Windows XP or the 32-bit Windows 7 version (at least 8GB or more if you are considering a 64 bit OS such as the 64-bit Windows 7 version). A 20" monitor is about the minimum I would consider for a machine; I would try for a pair of 20" or larger monitors if possible. For a video adapter, I would recommend a discrete solution within the last couple of generations (that would be the AMD 4xxx or 5xxx series of the NVIDIA GTX/GTS 1xx or 2xx). The motherbord graphics from AMD or NVIDIA might be sufficient for basic graphic editing tasks but the Intel motherboard graphics have been historically very bad (reviews indicate that the newest ones on the H55/H57 motherboards that go with the new Clarkdale chipsets are acceptable but not quite "good" yet).

An example system might be the Dell Inspiron 580s with the 22" monitor for $750. It has the basic features described above with the exception of a discrete graphics adapter, but it does use the current Intel graphics subsystem, which seems to be suitable for desktop work and light gaming. There are many other comparable systems out there, some for far less. The Dell I described above is an example of a system with current OS and hardware; going with a generation back such as the Core2 processor-based system with Windows XP or (shudder) Vista can result in significant savings.

It's always a tough tradeoff between price and features. When I set out to design installations like that I usually recommend $1200 per place to the customer, which includes the computer, a network port to plug it into, a desk to set it on, a license for MS Windows, a license for MS Office, and an anti-virus license. Any licenses for activity-specific software and for installation are extra. Plus the $2500 overhead for the file server machine and software.

The two biggest factors in user experience are responsiveness (directly related to how much RAM is in the machine) and display quality (monitor size and quality). If the monitor has awful color reproduction or low resolution then it's best to steer away from it. If possible I recommend actually going to a retail store and looking at a monitor before buying one, if possible. Don't buy at the retail store, but certainly go there and look at the monitor you're considering. Memory is cheap these days and it's getting harder to find a machine that isn't maxed out in terms of total allowable memory for a 32-bit OS. I had to work on a machine with 512MB the other day and I wanted to throw it out the window after 10 minutes of booting. $40 of RAM would have solved that problem.