cpu or gpu and mobo

[blitzcraig]

I currently have an athlon 64 3000 winchester running on a Gigabyte k8nsc-939 motherboard, 1 gig of corsair xms ram, and a BFG 6800gt oc. Would I benefit in gaming more by upgrading my CPU or my motherboard and videocard? Looking at the EVGA nforce 4 sli motherboard and the evga 7800gt for the gpu/mobo and the athlon 64 3700 san diego for the cpu. Won't be running in SLI.....not right now anyway...can't afford it. I am currently playing Q4 and FEAR. Which should I keep?