If you want good gaming results, try to save up some more money. Building your own is also a good idea, it will allow you to customize your machine more and save you money.
Thing you want to look for in a gaming computer:
1) AMD 64 processor (socket 939) AMD kills intel in gaming.
2) Good s939 motherboard, preferably made by ASUS, DFI, MSI, or Gigabyte. Make sure it has a DDR400 standard for RAM. Try to choose one that has SATA for hard drives. Also you want PCI express for your video card.
3) Try for a 1 or 2 gigs of DDR400 RAM.
4) You want a 7200RPM Hard drive, any less just plain sucks, and 10k RPM drives are very expensive. A SATA interface is preferable, because it alloys you to not need to deal with those annoying as **** ribbon cables with IDE.
5) Try for at least a 6600GT video card. That will allow you to play almost any current game on mid/high settings at a pretty good resolution. PCI express of course, if you get a PCI-ex motherboard, which you should.
6) A good solid PSU with plenty of amps on the 3.3V, 5V, and 12V rails. A good budget one for less than $50 (american, dont know canadian equiv) is Fortron. Make sure it has at least 450W, 500W+ is best. The next best step up and great PSU is the Antec TruepowerII 550W.