Anandtech has the new ATi X800XL 512mb video card in benchmarks. This new card has twice the onboard VRAM as the normal x800XL (256mb). See for yourself the performance differences between the 256 and 512 versions. Not that great and for the price ($449) not worth it really. A card like the Geforce 6800GT/Ultra or Radeon X850XT will get you better performance for the money, Full Benchmarks
That's an odd thing for ATI to do, you'd think they'd leave the 512MB stuff for their new chipsets really, not use a medium highend card. Doubling the ram makes the card a lousey deal as you said, normally it's priced very resonably, coming just under $300 @ newegg.
but those often only work for months, not even a full year. Or they fry the card on the spot. Very risky.