Right now I am running a system with a 7950 GT (512 MB) and was wondering what a good DX10 card be for around the same price that equals or beats the 7950GT in performance.

I would recommend at least a 8800GTS 320mb.

That's what I was leaning towards, but it has 320mb of memory, and I usually play at 1440x900 usually or 1680x1050, will 320mb be enough for those resolutions?

For the latter yes, just about.

I think 320MB would be enough for those resolutions.(At least for now!)