General upgrade rules are RAM-CPU-Graphics Card in that order. Then if your worried about Hard Drive space then the Hard Drive or if your looking for speed your looking for Solid State Drives. You could also upgrade your cooling situtation to higher RPM fan speeds or larger fans in general. Possibly even a new CPU Cooler or Memory Coolers along with Hard Drive Coolers it's really up to you because the fact you would know more then others whats going on with your computer.
As for the Single or Dual GPU question, I would always go Single GPU because the fact that the Second Card usally runs at a lower Slot Speed, but if your Mother Board does run at 16x - 16x then you will be fine.
Now cards are getting so nice and their running Dual GPUs on Single Cards that theres really no point of running Dual cards and now the FERMI GPUs are extremely advance and the CUDA Technology is really a neat thing.
Well I hope this helps you out. Any other questions just ask.