GP4 will run fine on that Compaq, but you need a better graphics card for GP4 to run super quick at higher resolutions with everything on max lol.
You might want to checkout [
www.aria.co.uk], they sell PC's where you can choose the parts........ Worth checking out...........
Does that compaq your looking at have the NForce chipset??? They should be avoided.........
Personally i think Time PC's and Systemax PCs (http://www.simply.co.uk) are quicker then compaq's..............
You might want to go for Athlons instead because they are cheap and much quicker then their Pentium 4 counterparts...... A lot of comapnies use Pentium 4 motherboards with the I845 chipset which is slow. Athlon garuntees lots of speed............. ;o)