Currently I've got a single 670, and a couple of hundred quid burning a hole in my pocket. I'm tempted to grab a second 670 and SLI it at the price scan have it on today only (£161) but I'd need a new PSU also (I currently only have 2 PCI-E connectors) so total cost would be around £240, less what I could get back for my current PSU (maybe £40). So total cost say £180.
That then set me to thinking - would I be better waiting for the aftermarket cooled 290s. Overall cost is likely to be similar say £320 for a card, and maybe £140 back for my 670 after paypal fees etc, I'd imagine performance is going to be slightly lower, but theres obviously less hassle with drivers etc.
I'm having trouble locating any comparisons though.