You could get one from ebay, Dell, or another online retailer. (I forget the links ) However, according to this site, the best you can get for the 8600 is the Geforece or Radeon 128mb.
Here's a guide with pics too.
Patch it, and if that does no good, see if you can disable HT.
Oh, and here's a site that has the Grim Fandango Soundtrack for download. I'm sure everyone who's played GF wants the soundtrack. Too bad they're so hard to find