The opterons are faster and better, however, also more expensive - in other words, get one if you can afford it, however, a 3800+ X2 is more than fast enough.
Why not give the 920 a go? I have an 820, and it goes ZOOOM - however, the 920 is a little cheaper than the 3800+ and performs on about the same level - the 820 being a little bit below both the 3800+ X2 and the 920.
As for going SLI/Crossfire - there isnt a lot of point when your only driving one display, and at 1280x1024 at that. a 7900GT or whatever you showed is more than fast enough, and if you get multiple displays in the future, grab an SLI board, and stick in another one. That simple really - SLI boards are now cheap - like $130AUD (dont know, dont care what that is in USD). Have fun now.