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.
With out knowing what videocard black fire fox has its hard to tell. I was using the onboard intel 900 video which scored 5370 in 3D mark 01. After installing the 7600GS I scored 16718 and after overclocking it a bit I was 17243. In my case I definately saw a benchmark peformance increase.
EDIT: Ok he has a 256mb card already I re-read the original post.
Then No there probally won't be a big increase. Not more than 3-5 % if that.
Since you are a stranger in the rankings it may help if you put your computer info in your signature. If you are running an older system (2yrs or older) you may want to save your money for a new system and use what parts you can from your old one.