I want something that could support various games like Gmod and Oblivion.
gmod is JUST half life 2. thats it. half life 2 is basically an old game. looks ok, but i wouldn't consider it a high graphic game anymore.
oblivion likely wont run with high settings on a 400 dollar computer. no matter what parts you use.
and if you ever have plans to play modern games at all. you need DX10 now.
even if it is JUST a 500m video card. DX9 era is far behind.
DX10 capable hardware requires 64bit OS (i believe, at least to run normally well)
and that means all around slightly better hardware then you are willing to pay for.
i only bring this up because you clearly show interest in being more then a casual gamer.