Either stick in a cheap TNT2 type card and wait, or else get a 4200-128MB, it's fast and won't devalue as badly as other cards, hence will be easy to sell on.
As for 128MB being in transition, any 64MB card is NOT a smart buy for any new card, not only do some current games suffer with 64MB but in 6-12 months any 64MB card will be VERY hard to sell on, just try selling a 32MB card now! 256MB however is pretty pointless, it will be well over a year before you'll get any appreciable benefit from it, and by then you won't have to pay such a price premium to get one. Unless your budget is very tight then acoid 64MB cards, unless you're loopy forget 256MB ... get 128MB.
I think ATI are so pleased with Rad9700 perf that they feel they can just wait for nVidia to release their new cards before making the Rad9700 available ... LOL!