Okay Nick a few ideas for ya here:
1) 7800 is fast enough to play todays games and has enough extra features to cope with unreal3 when released. based on that it will future proof yoru upgrde for gaming for a while.
2) A 6800GT in SLI will probably be good enough too.
3) For benching you will need two looooooooool
4) Given that the 7800 in sli is CPU limited and we have yet to realise it's true power - upgrading to a 32pipe unit adds no value until dual core is fully supported in games (12 months away maybe sooner for a few) or the physics chip is supported to relieve the CPU bottleneck.
Point 4 is assuming dual core support does a SLI/Crossfire type performance increase of 78%+ in games
Thats why i decided to invest in some 7800 magic (and then have one blow up on me looooooool)
The choice i yours mate but probably worth gettting a 7800, maybe buying another next year to go SLI, with you having dual core already I am guessing you are in good shape for 2006.
Mav