Budget Gaming PC for SC2, WoW, DA:O

So the video card in my dated gaming PC finally died, and now I'm looking to build a new system. It will be my first build. Luckily, I can salvage some parts from my old system.

Approximate Purchase Date: Soon. May be forced to buy some parts now and some parts around Xmas.

Budget Range: $600-750 Before Rebates

System Usage from Most to Least Important: Starcraft 2, World of Warcraft: Cataclysm, Dragon Age: Origins, Audio Recording

Parts Not Required: I'm hoping to reuse some parts from my old comp, but not sure about some of them. I have a Razer Lycosa Keyboard, Microsoft Optical Mouse, 23" Widescreen Monitor, 2.1 Speakers, Headset w/Mic, and a Delta-44 PCI Sound Card that I plan to use for audio recording. Also have a Speed-600ATX 600W PSU and a slightly damaged mid tower case that functions, but ideally I'd like to replace. The DVD-ROM and CD-RW drives from my old comp still work fine, but use PATA. I also have two hard drives, one 40 GB and one 160 GB, both PATA.

Preferred Website(s) for Parts: Newegg.ca, TigerDirect.ca, etccomputers.ca

Country of Origin: Canada

Parts Preferences: Prefer an Intel CPU (likely an i3) as they seem to give the best results in SC2, but willing to accept AMD if the price is right and the frame rates are playable. I'd like a case with washable air filters if possible (was looking at the Antec Three Hundred Illusion).

Overclocking: No

SLI or Crossfire: No

Monitor Resolution: 1920x1080

Additional Comments: I do not need to run these games on max settings. Medium settings with stable frame rates is what I'm aiming for. I'm not concerned about future upgrades, as I should be in a position to buy a new comp with a less stingy budget 2-3 years down the road. I'm not sure if my old hard drives will cause any gaming performance bottlenecks if reused. USB 3.0 would be a plus, but I can live without it.