Well unfortunately it's hard to find replacement parts, and while you could find a replacement unit for the Gf6800 it might not be worth the hassle, but it is doable, so don't write it off completely.
Best way is to contact DELL and order a replacement part. If you want to upgrade there's a few vendors, they're just trickier to find. Dell often has a proprietary connector though so make sure if you buy from someone else it's a) compatible, and b) you can return it if it doesn't fit.
There's a guy at notebookreviews.com who had a list, but I don't remember his username, he had an extensive list from both the green and red teams. One place I had bookmarked used to sell more, but looks like they're slowly going out of business with only the MRX1300 and MRX1900 for sales (they had issues with their GFGO7900s);
http/www.mxm-upgrade.com/store.html
It's not an easy procedure but it's doable;
http://www.laptoplogic.com/resources/detail.php?id=31&page=3
http://www.legitreviews.com/article/349/1/
Overall for your needs and budget, I'd recommend looking at the ASUS G2S.
There are other models out there from other companies, and if you don't care about DX10, then maybe look for a Napa based laptop with a T7200-7600 and a GFGO7900/7950-GTX.
EDIT: Vista is having growing pains, and unfortunately has things like DRM bloat to it so many people, myself included, just don't want the hassle until either SP1 or some compelling reason like Crysis comes out that makes us want to switch.
Some companies are allowing your to have old XP on it, so check before you buy. Personally I'm going to get the cheap Ultimate version when I buy, because I already have XP, and then just make sure it has support for XP before I buy, and then blow out vista at first, load old XP Pro, and then when I want to use Vista, then just use the disk that ships with it, and then recover the install and then go from there. Best of both worlds at the cheapest price. If you never plan on going to Vista, and you already have XP, then try to get them to put Linux on it instead, but make sure they'll give you support, because that's one of the things you pay for in the OEM package with the laptop vendors. Sometimes it's just less hassle to have all the drivers/etc pre-configured by the mfr and then no hassles. Just depends on your comfort level.