I have a 7600GS PCI-e 256mb. The main reason I choose it is because I only have a 300W power supply. What power suppply do you have and what amperage on the 12V rail does it have. The card you are looking at only requires a 300W powersupply.
If the above 2 posts about the card not being able to really utilize the extra memory I would just go with the 256mb version. That is a new one to add to the books. Its just like back in the days of external modems when you needed a 16550 UART chip on your serial controller to sustainan a 14.4 or higher baud rate. A little tidbit of info that made a lot of difference back in the day.