i would wait and build a completely new system some time next year, thats what i am doing.
Next year heaps of cool stuff is being introduced, it will be a great year for computers!
Windows Vista will be released - a new operating system that fully supports 64 bit and multi threads. Also it puts all the work of rendering the desktop to the gpu instead of the cpu so your brand new gpu will be used in things other than 3d games.
Dual core will become even more affordable next year. It is still quite new and expensive, next year it will be more adopted and prices will drop. Also other programs and games will be released with multi thread and 64 bit capability so things will be even faster.
Also, AMD (which is a big favourite in this forum) will be releasing socket M2 which has DDR2 support which lets you have higher bandwidth, and by then the latency problem should be fixed up.
Core 2 Duo E6400, DFI Infinity 975X/G, 2x 512mb DDR2 667mhz, Albatron 7900gt, WD 200gb SATA, Samsung DVD-RW, Silverstone ST-50EF 500w PSU.