As far as I know, they go and check your receipts. You could always try, worst thing they'll do is deny your claim, no worse than the situation you're in now.
I don't think moving from a 8800GTS to crossfire'd 3780s would be a big enough increase to put up with the crossfire headaches. I'll wait for the 4xxx series or wait for G200 (Or whatever they are calling it now)
I've heard lots of complaining about SLI but I rarely hear bad stuff about Crossfire, but SLI needs to fix some stuff like scaling (well this doesn't apply anymore) support for dual video cards with both cards enabled
As far as I know, they go and check your receipts. You could always try, worst thing they'll do is deny your claim, no worse than the situation you're in now.