Basically everybody is correct. Your PSU is not good, but a 7750 consumes very little power. It should run the card just fine assuming it is functioning properly. No one around here thinks it's safe to run an old, inexpensive PSU like that with a nice new card. But like I said, if the PSU is functioning properly then it should work.
The best advice would be to get a new PSU (Antec, Seasonic, Corsair) to go along with your new 7750 and carry it over to whatever new system you eventually build. Obviously your cpu/mobo/ram are outdated, and sometime soon (when you get the cash) you will likely want to upgrade them. A quality 400w will suffice, but for longevity and upgradability I would go with a ~500w.