I'd recommend a 4-core upgrade. If you want to stick with AMD, go with the Phenom II cpu. Otherwise, the Intel i7 920 is great for bang-for-buck ratio. An upgrade like that will require a new motherboard (unless your board supports AM2 and you stay with AMD) and new ram if you go with the i7 920 (6gb will do nicely).

If those options are out of your price range, check if your motherboard supports 8gb memory and upgrade that aspect.

The video card would be my final recommendation, below the cpu and ram. Since, it is not a *horribly* outdated card... but outdated nontheless. Though, cost for improvement may meet or exceed the cost for improving ram (if not uprading the cpu as well).

That's about it. You may be wondering if SSD or velociraptors in raid 0 are worth it... but they are not. You have too many other areas for improvement that will yield much greater results with less money invested.