Over your current setup that would help loads, but there is a world of difference between the 6800go vanilla and ultra (only go for it if you can justify the cash though! and you might want to consider a 7800gs in that case).
You can order it directly form dell spare parts, Part # W5373. Though that is the most expensive route, not sure of its official price.
Alternatively search around forums with lots of dell users, i saw one for sale over
here for $150, not sure if it sold yet, but ask around and someone will be bound to have one.
And again you could search on ebay, you should expect to pay about $150
Dan