All you need to do is Pick up a C2D board and CPU and some DDR2 ram and you will have a nice system.
I suggest upgrading when the C2D price cuts come out with an
E6750
Gigabyte DS3R
2x1Gb G.Skill HZ
Your looking at around a 430-450 buck upgrade but your 7900GTX is still a nice card as well as the Antec Nine Hundred is an awesome case. The HDD is an HDD and it won't help you too much but you could grab a 250GB SATA 3.0 16MB cache for around 65 bucks.