Why not something like
http://www.acerdirect.co.uk/Acer_Veriton_M670G_Desktop_Tower_PS.M6GE1.U02/version.asp?PID=406 with a mid-range nVidia [or ATI... as you like] gfx card?
I prefer Intel CPU's with Intel-chipset-based motherboard.
Maybe you can save some money looking for a FreeDOS or Linux preinstalled OS... if you do not need a M$ OS...
Assembled machine gives you the freedom to mix hardware to suit your needs (do you really need a QuadCore machine?)
a base would be:
Asus P5Q3 MoBo [up to 1333MHz FSB]
Intel Core2DUO or Core2Quad CPU [Boxed]
3Gb Kingston/Corsair 1333MHz RAM (unless you have a 64bit OS you can't see more than that)
ASUS Nvidia GF9600GT 1G DDR3 HDTV 2xDVI PCI - E
Pioneer DVD burner
500Gb SATA-II HDD
Midi-tower w7600W PSU
more or less you will pay a fair price for some good hardware... (maybe I would consider a smaller CPU to save money: say core2duo in place of a QuadCore)