If you are already having the C2D processor and DDR2 ram and don't want to upgrade to new rig now then my suggestion will be:-
ASUS P5G41T-MLX3 @ 2.4K. If you don't have any middle end graphics card or not planning to add one in the rig of yours, then get anything 500W or above from Intex, Frontech etc. Price will be around 0.5K