Good way to look at it so far is that as soon as there is a new xxxMB gaming card available under two different brands, which really only is ATI and nVidia these days, then chances are you should go for a card with half that if you absolutely need a new card. Now that there are 256MB cards you only really need 128, back when 128MB first came out on cards like the 8500s, you only really needed 64.
Although, need is based on whether or not your gaming with new games, if you're a dedicated gamer to one or few specific games and you're still playing an old game like CS and are just looking for increased performance there, then no, 128 isn't needed at all and won't help really one bit other than that all of the fastest boards today have at least 128MB of ram so unless you want to play an old game with like 6X FSAA and 16X AF then maybe you might want to look into a 9800 Pro with 128MB+ of ram but I'm just babbling now so...whatever.