G,
Get Name Brand, HP, Dell are about all that is left in the desktop world.
Windows 7 64 bit Professional
Core i7 or i5 processor
8 GB Ram or more
2 500 GB drives mirrored raid Larger if price permits
CD/DVD Player/Burner
You might consider running dual monitors so that you can have an app open on each screen. We run dual monitors here at work and if I tried to take the second monitor away from my users, I would get my hands cut off. Running two 19″ or 20″ standard 4:3 monitors works great. The newer 16:9 monitors are alright but in my opinion you need a 22″.
If you are leaving that PC on 24X7 365 it should be in a cool area especially in the summer time when it can get really hot on long holiday weekends.
If you have any questions feel free to pm me.