Sorry for the absense guys, internet went down
I wouldn't recommend using any Socket 939 setup for a Garry's Mod server. You'll NEVER hear me NOT recommending a 939 again....but Garry's Mod needs TONS of processing power. You need to go with the highest clocked C2D or C2Q you can get (or one that overclocks really high). Garry's Mod will only use one core unfortunately, and if you have a lot of physics objects (I know I do), then you will certainly lag the server down quite easily.
I would recommend you either search around and find a really good clocking Q6600, or on a budget, go for an e5200 build for your game server. I mean it, you need TONS of processing power from one core....although I'm sure you already know that.