Straffo,
You're pretty high up on the scale with the last gen hardware, so you're probably only going to see big gains with big bucks.
I think you'd be disappointed with anything less than a 6800 or 6800GT vid card, and if you go for a new cpu and mobo you'd be crazy to not go big for a socket 939 nforce4 PCI-x solution so you can take advantage of the new dual core cpus when their price drops.
If I were you, I'd hold out until you can get a dual core A64, and then go big. You're just not going to get much improvement without spending a bunch of money. You could get a new vid card or cpu, but neither will transfer to your next system.