You basically need to identical cards.
SLI is going to need to have the same GDDR capacity if you want it to work .
Put in any other companies 9500GT 1GB card and it'll work, mixing 2 different capacity cards is a no. We usually avoid mixing companies too, but , it works in most cases with the same GDDR capacity.
If you use 2 x 1GB cards you get a graphics memory of 1GB, if you use 2 GPUs of different memory capacities the rig will not be able to fix the total amount of GPU memory it has.... since it's not mathematical addition.