Hmm. The cards are actually extremely similar in performance. I would try the 7300GT just to see if the 512mb helps at all.
JDFan is correct in Nvidia's naming nomenclature, but in addition to all that-- GT cards are usually clocked higher than GS cards... So i think that in addition to the ram on 2 monitors evens them out.
Tom's Hierarchy chart puts them in the same performance bracket: http://www.tomshardware.com/reviews/best-gaming-graphics-card,2544-7.html