I dunno... Maby current heavy games, or future games will make use of the extra ram, but atleast when I was still even remotely intrested in playing (over a year ago) most benchmarks showed a very minor differance between 128 and 256MB.
I would have a look at something like tomshardware vga-charts and see what the situation currently is, and if you find any games that show a big differance you can trust there will be alot more to come. If nothing seems to benefit alot then you might be better off putting the money on something more usefull.