new cpu or gpu?

[GastricTitan]

I currently have the following build

radeon 2900 xt
g.skill 4gb ram
intel core 2 2.33 oc at 2.8
windows vista 64

I'm trying to get a bit more juice out of my setup to run crysis and UT3 at a more stable framerate. What is better to upgrade, the cpu to a 2.4 quad or get a couple of radeon 3870 xt's and crossfire?

Thanks
---