256 megs will give a very, very small performance advantage when running at unholy high resolutions with Antialiassing.
It also gives you the option of using high resolution textures in next-generation games, but the only game in existance that I've heard of making use of more than 128 megs of textures is half-life 2.
Basically, 256 megs is a complete waste right now. 128 megs is more than ample, hell, 64 megs will do just fine for some time...
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Radeon 9500 (modded to PRO w/8 pixel pipelines)
AMD AthlonXP 2000+
3dMark03: 3529