I agree, unless you're talking about 100 Yen or Rupees! Sure, it's nice to get the 128MB to make the card last longer but not at that price, just enjoy the money elsewhere, like on some games or a new CPU.
i just go the PNY Verto Ti 4200 64mb version
All i got say is it pretty much rocks. Its over clocked slightly(look at my signature) but for the most part you really dont need to overclock it. i ran 9790marks on 3dmark2000se and overclocked i got 12987...its a good card. well made too
Definitely the 128MB card. 100 extra is not a lot to pay for considerable performance benefits in most modern games. The 128MB version is clocked lower by default (and uses slower rated RAM) but this is outweighed by the advantage of having a larger framebuffer in many situations. You can overclock the memory to 500MHz+ fairly easily, anyway.