Well Ima big HL2 DM, and Gmod 9 player. And to say the least, on my DirectX6 capable graphics cards.... Its laggy. I'm wondering how much it would cost to get it running "average." I would want at least a DX8 capable card and a cheap, yet fast processor. I am also deeply ineed of more ram. Running HL2 Garrys Mod on 256 mb of ram is... Depressing. I often get crashes stating "No Free memorie" or of that sort. Also is a 360 kbps Down, and a 160 kbps Up max, be an acceptible conection to play WoW? When I play Gmod my ping likes to wander around 70 - 130.
What's the specs of your system as it stands now? A faster CPU isn't bad, but gaming, particularly FPS games, are more graphics intense. If you rebuild the box and are willing to piece it together yourself, you can do a new box for under $600 easily, maybe $500 depending on the route you take.