I have a 9800pro along with 1GB of RAM and an AMD XP2600 mobile CPU, which is overclocked. So my system is similar to the OP's.
If you have a read of this thread...
http://forums.overclockers.co.uk/showthread.php?t=17487487
...it will link to this VERY informative thread regarding the need and benefits from upgrading the CPU or the GPU. It also gives examples of upgrading from 1GB to 2GB of memory...
http://www.firingsquad.com/hardware/fear_cpu_performance/
You will notice that there is very little difference from a AMD 3000+ to a 4800+ that can be attributed to the change of the CPU. There is no difference from 1GB to 2GB of memory. Nearly all of the performance differences noted is seen when the graphics card is upgraded, supporting the thoughts of the OP.
However I see the Civ series (but Civ 4 obviously demand more from your graphics card than the earlier ones) a little differently as most, but not all, of the lag noted can be the computation of the AI's turn which surely must require a faster processor rather than a faster GPU...? I can understand that Civ 4 will up the requirements of the GPU and, of course, memory but I would still think that the CPU plays a vital role in the playability of the game over that of a FPS type of game that they are comparing in the second link that I posted and certainly more so than the synthetic benchmarks that you can run, like the 3D benchmarks.
I hope that the OP is able to take some timed benchmarks of how Civ 4 is running doing certain things and IF they can be saved and reproduced when the 6800 is bought to see just what has changed.
In TBS (turn base strategy) types of games I think that the GPU, depending on the graphics, textures. lighting etc etc etc and resolution used, has a part to play but the CPU and memory is more important. Not having got the game yet I might find though that Civ 4 is pushing both demands for a fast GPU and CPU.
I hope that Orion66 keeps us posted on his results, and good luck with your upgrade
EDIT: Just remembered that my friend upgraded his graphics card a while ago for his socket A based rig to an ATI 800 XTPE and his games played absolutely superb and were just as fast as those using a AMD64 rig BUT he only played FPS type of games, like Counter Strike, Far Cry or Half Life 2.