so it's not worth getting a core 2 quad? if not, i'll just opt for the video card alone.
Shop around before giving up. The cheapest I was finding Quad Core's on 775 was ~$200 (new). For that price it's not worth it. If you want to take the chance on a used CPU I'm sure you could find them fairly cheap on eBay, but that's entirely up to you.
The problem is that the 775 chipset has been dead for quite some time. You're better off upgrading everything to 1155 or 970/990 now or wait until next year when the new Intel's drop (Haswell).
But, by all means, feel free to upgrade to an HD 7750. While the 7750 doesn't take any additional power from the PSU like a 7770 would, I would still look at upgrading your PSU in the near future. The units that come with pre built systems have just enough Amps & Wattage to run the computer as it is.