[Build Help] Best video card for me?

I've wanted to build a new computer for a while but don't really have the money to build what I want. A friend of mine gave me a SSD for Christmas though and now I'm thinking I will see a real performance boost if I just swap hard drives and replace my video card. The new hard drive is the OCZ Agility 3 SATA III SSD 120GB, and here is the rest of my hardware:

AMD Phenom II X4 955 3.2 GHz Processor Biostar TA890FXE Motherboard 4 GM DDR3 RAM 750W Power supply NVIDIA GeForce 9800GTX+ Current video card

I'm not sure what other pertinent information there would be, basically I'd like to run Skyrim on better than Medium which is where I'm at now and sometimes the video card crashes out when I play TF2 of all things. I'm shooting for something around $300 I think and any and all help is very much appreciated.