In general you will not see much more peformance difference because most games do not use that much video memory (crysis may get close though). GDDR3 is a bit faster than GDDR2. This wikipedia article will help explain the differences much better than I can on here:
http://en.wikipedia.org/wiki/GDDR
Manafactures like to put extra memory on their graphics cards because it is cheap (especially GDDR2 over GDDR3) and people will think it is better. If I was choosing between these two cards I would most likely go for the 384MB version.
I hope this information helps. Good luck.
