I would suggest getting a Dell Inspiron 530. It comes with the Q6600 quad core. You will however need to upgrade the GFX card, ram and PSU but it would still come under $800 and might be easier in your case.

I've got 2 of them speced out the same way (except one has 4g ram while the other only has 3) and I've had no issues running 5 clients on either.