i have a system, not very similar but i was in the same boat
x700pro, i now have a 7800gs..
for bf2 id say sure get the extra memory, and then maybe a 6800gs, or a 7800gs depending on ur budget. if ur budget is big, say 1200. id upgrade the entire system. but if u want a fairly cheap upgrade a 6800gs will run oblivion on medium/high (no hdr, my friend has a 6800gt with amd 3500, proc isnt a bottleneck so i assume ull get similar frames as him) get the other gig of memory if u want to run bf2 on max settings. i only have 1 gig and can run it smooth enough for me, but its really personal preference. also the 7800gs in oblivion ROCKS. constant stable fps with HDR enabled on not max but pretty high settings..
so in simple terms since i can be confusing
6800gs + 1 gb memory
these will all offer performance increases over ur 8 pipeline 9800xt...
i was the same, didnt wanna upgrade (sincei had jsut got the x700pro) but once i played oblivion, i knew i wanted to play that game at higher settings... it just adds so much more to the experience. ill admit in sme cases (when ur in oblivion gates) it slows wayyyy down, like 25fps... but thats because theres lava (water) effects all over the place... normal rivers and ponds dont do that though. id say if u do get oblivion and dont upgrade, dont even try to run the game on even close to higher settings, because it will make u want to upgrade.. for bf2 anotehr gig alone will improve gameplay, but an x800gto IMO will improve gameplay in all games over 2 gigs of memory.. wait if u want (id say get a conroe because they seem to be the best processors until amd goes 65nm, which wont be until 07). anyways, good luck