Nvidia cards tend to not have a whole lot of overclockability with their stock coolers. If you want to run them with a water block or some sort of massive heatpipe attachment, then you can crank them up faster, especially some of the evga ones, but for the most part the AMD ones have a higher heat threshhold these days. that wasn't always the case. I second the AMD / ATI drivers still sucking, and I for one remain convinced its primarily because ATI has idiot programmers rather than any sabotage efforts on Nvidia's part. I tend to go wtih Nvidia cards for the most part. I have a pair of 580 GTX 3GB cards at the moment in SLI mode, about 15% slower than the 680 GTX, but I got them new at the time for a very good price. Watch out for some of the 650 series GTX cards, they cut out half the ram channel banwidth apperently, even with faster ram its still ends up being a slower card than an equivilent 550 gtx.
Most, though not all, of the x58 and newer intel chipset boards will also support AMD crossfire. Assuming the board has 2 PCI express 16X slots to begin with. PCI express 16x 3.0 is definitly worth it if you have the motherboard hardware to make it work. You lose about 10-15% throughput going from 3.0 to 2.0, but the cards will handle it fine. It becomes more of an issue at SLI or crossfire interface speeds.