Nothing wrong with GFX with dual 6800 Ultras, they're pretty good cards. If anything, the Ram is the weekest link, but shouldn't cause a problem.
Doom3 was built way ahead of it's time, and highest settings are very demanding, so don't be supprised. Some games, despite being old graphics, are just badly made, so don't be too hard on that.
Head here,
http://www.futuremark.com/download/
Get 3d Mark 03 Free edition, and see how good your PC is, (don't get 05, that's just depressing). I would suggest a score of 10K +
I don't see how the power would be a slight issue, would relate to a major issue maybe. Do the usual defrag and stuff, and scan disk.
Make sure you have 5Gb free HD space at all times, and do this.
(XP)Right click my computer > Properties > select the "advanced" Tab > next to performance, select settings > Advanced tab > under virtual memory select "change" > Next to initial size type "2000" and next to max size type "4000", select SET, > OK x3
Let us know how you get on
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