Length of the card(if your case is small), How many Watts it uses (the more watts, the bigger PSU you will need), core clock speed/Boost Clock speeds.(the higher the better), The amount of Cuda cores/stream Processors(the higher the better) and the the size of the bus(192 bit, 256 bit, etc... The higher the better)
Also, don't pay anyone to put that in for you, do it yourself, it's the easier thing to install. It just slides right into the PCI slot. then you just plug in the 6 or 8 pin adapter from your PSU if it needs it.