I would go to Circuit City or BestBuy and get whatever they have around $500, ideally something that happens to be on sale. Just make sure it says Core 2 somewhere. You could build your own PC yourself from parts but this way it's cheaper and safer and faster and you get a warranty.
Or check one of the threads with titles like "help me pick parts for a $700 build"...
What OS have you got? If it's Vista you do need good video.
What software have you got? Just curious if it's something that works best on a quad-core processor.
How much money are you willing to spend on the PC itself (don't include monitor, speakers, keyboard, mouse)?