You do not say what resolution you are trying to play doom3 or far cry in, or what the rest of your system specs are.
Also Doom 3's ultra quality mode is really designed for graphics cards with 512mb of memory (although it does work on most cards that have 256mb okay).
The main difference between the 6800 and the 6800gt or 6800ultra, as stated is the reduced number or pipelines from 16 to 12, but also the reduction in memory bandwidth as the gt or ultra use GDDR3 at 1000/1100Mhz
whereas the 6800 uses DDR at 700Mhz.
3000 marks in 3dmark 2005 is a decent enough score, since 2005 is a pure directx 9.0c test and really stresses most cards.
You may be able to "soft-mod" the 6800 into having 16 pipelines instead of 12 by using RIVATUNER, or alternatively there are some guides to "hard-mod" the 6800 now available, but as always this may or may not work.
I personally would stick with the 6800 for the time being, and wait until the next generation of graphics cards before upgrading.