if you get the x1900xt or 7900gtx you need 2GB of ram for bf2, period. and at this point you would want to go the new intel conroe. personally for gamig i think your current cpu is fine as the games are not cpu limited but gpu limited. for the $65 for the m/b and ~$140 for the 2GB pc3200 along with the x1900xt you would be fine. i would say that your current cpu would take you through anothe year of gaming before it became an issue, then just pick up whatever processor/ram/m/b is the best bang for the buck and go that route.
for gaming, and all i game is bf2, i went from a p4 2.8c w/ x800xtpe to a opty144/nf3/x800xtpe to a opty144/nf4/x1800xt and honestly can't tell the difference besides turning up the eye candy, but i am limited to 1280x1024. also there was no visual difference from the cpu standpoint between the p4, opty @ 1.8GHz and opty @ 2.5GHz although there is a differenc when benchmarking. basically the p4 2.8c = the opty because they had the same gpu and i could keep total ram usage under 1GB, the x1800xt uses a lot more ram even if the settings in game are the same(like 500MB more, don't now why), thus i needed to upgrade to 2GB of pc3200 as soon as i got the x1800xt, which was unexpected and honestly i probably woudn't have gone the x1800xt if i knew i would need to go to 2GB.