I will be doing gaming, but at the same time I will be running an MC server. I do plan for dual/ tri sli down the road. I think I should be able to handle a good many games as the 670 I am getting is the EVGA FTW+ 4GB edition. The card is running at close to 680 speeds. I thought the extra Vram should help but I digress.
I always lean toward the fastest single card you can afford, at least initially. SLI is getting less and less painful but still has issues compared to just a single card. I keep it simple, since I'm stupid.