OK, I'll go based on just the games you provided. Though, actually, if you go to (http://www.escapistmagazine.com/forums/read/9.262808-For-the-non-PC-gamers-out-there-here-is-a-cheap-gaming-rig-now-with-EXTREME-budget-edition) that's actually quite a cheap and good load out to compare against (don't buy it, though, because if you're anything like me, the actual build will be hashed up and software conflict will be the least of your problems).
CSS: any DualCore processor will run this along with DirectX 9 graphics cards (ones that are 256MB memory) and I won't bother going into RAM (less than 1GB required, so go figure). Because it's quite an old game, most new release mid-range (or even cheap) PC's these days will run them.
And it's pretty much the same story with TF2.
Don't know what the Canadian dollar/GBP rate of exchange is, but GBP500 can get you 4GB RAM, 1TB hard disk space, GeForce 310 video card and a 2GHz QuadCore all of which is more than enough to play a looooooooooooot of games. Hope that helps