Yes, a radeon hd 7750 is good for under 120, but you will need a new power supply (maybe 500w) and possibly a new motherboard.
From what I've read, I don't think I can upgrade the dell motherboard because dell configured it weirdly on purpose. I do not have enough money in my budget to buy a new case and build a custom PC at the moment. Are there other cards available? Like the 6850?