What the hell are you saying?
If you meant that if you get CSS and Gmod, yeah, you can do that. If you want to use HL models/shit, you need either HL2 or HLS(HLS and HL is different, one is original, one is source).
If you get the Orange Box for the PC/Steam, then it will be good because you get to use TF2/Portal/HL2 content in lets say, a CSS map.
About the computer:
I don't know much about AMD or Intel processors.
If you plan on using ASSLOADS of custom shit in Gmod, 320GB is good, some of them could be for porn.
2GB RAM is alright, but you might need 4 soon, for less lag.
XP is good, but some people are starting to like Vista more.
NVIDIA is good, but you need an 8 series card+(8000-9600) or one of those new video cards.
Overall, it might be able to run Gmod on high with good fps.