Couldn't you OC a 3500 to the specs of a 3800?
Yes, you could even OC it to that of the 4000 if you wanted to. This is on air cooling as well.
I'd be cautious about getting a prebuilt system. If you want to add a second 6800GT, you have to know the EXACT manufacturer and the EXACT version. You can't just put ANY two 6800GTs together for SLi- they MUST be from the SAME manufacturer and the SAME model.
Sometimes those prebuilt systems feature "custom" vcards, not made by any major name brand, so to speak, or only made for that specific computer.
I'd go with a custom built rig if I were you. Don't be afraid to build the system on your own. It isn't hard, so long as you follow directions. And it is more difficult to "fry" your system than you may think. I guess you could say it is near idiot proof, though I mean this with no offense.
Try to build it on your own and save yourself some cash.