Anything from AMD more powerful than a HD7750 will require a six pin adapter, which I am sure the OEM psu does not have. I know there are molex to six pin adapters, but it might not even have enough free molex connectors to use the adapter, and I would hesitate to put that much load on the OEM psu anyway.
The 750Ti is great for an OEM system, but somewhat overpriced. I think it would be better to upgrade the PSU and try to find a video card that is a better value. Right now on NewEgg an EVGA 430W with 34 amps on the 12V rail is only 29.99 or 19.99 after rebate. This would allow for a much broader selection of gpus, plus would give more peace of mind that the psu is up to the task.