You don't really need a quad for gaming, but really depends on what games you play and @ what resolution.
Your system looks pretty good to me apart from the GFX card.
You did'nt give your budget, but upgrade to a GTX 460 (Fermi)
& that should be plenty good.
If you wanna save some money - Radeon HD 5770
I think upgrading the CPU is not totally necessary, but if you really want - Core 2 Quad Q9650
- one of the best, but highly overpriced quads.
I'll say kepp the 8400, get that GTX 460 & keep it at that.