My Friend has a Geforce GTX 650 i know the card cost him a little over $100. i currently do not know his bored manufacturer, but he can run Crossfire and his current CPU is a quad core AMD his bored is 2 yrs old roughly.
he has a 9800GTX and ran starcraft 2 on high. i believe possibly extream graphics. but had it on high he so claims.
with his setup and currently running a Geforce GTX 650 and playing starcraft 2 he had to have his graphics on low and he would get the frames normally you would get. any higher it would be choppy and lag. he then told me now he can run on medium because he discovered on his bored he turned on a option of "Enable Core". anyway i see that graphics card he should with starcraft 2 at least be running on High Graphics i believe if not extream. his card can do SLI, but his bored does Crossfire. could this be because his card is not really compatible with his bored???
oh and the card is a PCI E 3.0 but he only can support PCI E 2.0 i know there backwards compatible. it might be an issue of him loosing performance on a pci e 3.0 card plugged into a pcie 2.0 slot... would like some feed back on this.
There would be no impact of performance the graphics card you mentions won't have enough thruput to even max out the PCI-E 2.0
It's most likely a driver issue or motherBOARD issue. The gtx 650 is almost as good as a gtx 460 which can run sc2 maxed out.
Same what TropiKo said, do a clean driver install of the GPU to try if it solves the problem.
Maybe a motherboard issue.
A GTX 780Ti is not even a card that can bottleneck a PCI-E 2.0
Do you have CPU and motherboard specs ?