why lower memory MB is better?
Less memory isn't better if it is clocked the same. BUT, it is very often cheaper. And 512MB is typically going to help most at higher resolution and/or with FSAA or with massive textures (says Oblivion texture mod). And even the 2600XT isn't going to do very well in those cases. That is why he would recommend the 256MB card....save you money. If priced the same and clocked the same by all means go with the 512MB card. But if the 256MB is a fair amount cheaper or it is clocked higher( say GDDR3 in the X1650,s or 2600 pro), then grab the 256MB card.