1. No, if your sticking with a memory bus of 128-bit, it won't gain you anything, except less money in your wallet (if that's your aim)...
lets you store more textures
2. No, you won't be able to play at the high resolutions or with heavy image settings where 256MB may have an effect...
3. Yes, it sure is...
If you'd be willing to spend $ 119 (after Mail-In Rebate), I can recommend you a
GigaByte 6600GT, given that your mobo supports PCI-e...
Otherwise, $ 119 (after Mail-In Rebate) for an AGP
eVGA 6600GT...
At these prices, those are the best value for money nowadays, and will provide you with a lot more gaming power than regular X700's or 6600LE's...