Hmm, well, changing the bios to XT does accomplish more than "nothing" on the 128 cards. if 500 extra points on 2003 is nothing, and better performance with FSAA is nothing than I guess your right. However, the bios update allowed me to increase my ram frequency from about 350 average to 400 average, for an increase of 100 total (700 - 800) This in turn increases FSAA performance, as well as other ram intensive operations.
I also heard that it unlocks an optimized 2.0 shader on the 360 cores, but what do I know.
So in turn, 128 megs is fine for compressed textures (which have just about zero difference in quality from uncompressed, ie you can't tell the difference) and any gaming resolutions below 1600 - 1200 (does anyone really game at that res anyway?) will have no difference in performances between the two cards.
As for being disapointed in the future, how long are we talking about? Eventually all 9800 cards whether 256 meg or 128 will be out in favor of the X800's... So no, I wouldn't pay more for the 256 meg version. If you can get it at the same price go for it, but I doubt you can.
As for better performance in HL2 and Doom 3.... if you really think that the extra ram will give you more options your kidding yourself. the 9800 pro/xt won't be able to keep up with any settings that will require that much ram anyway... Only the X800 and similarly high performance cards will be able to take advantage of this.
I know I'm new here, but take this advice, if you can get a 9800 pro cheap, regardless of ram amount, do it. especially if you can get a 360 core (more oc-able), Sapphire / MSI / BBA all generally use the 360 core on any card you'll find on the market. I can't say anything about the other companies because I don't have any first hand experience with them. However, I build computers as a side hobby, and have about 130 systems under my belt, if that's any form of credential.