It works very well for me on an 8 core i7-965 or 4 core Q9450. Make sure you have done the following as its a little picky due to the way Bethesda coded it:
1: Works best installed on your Windows drive letter (especially if you have bought the DLC). If not on your windows drive letter uninstall & reinstall on your Windows drive.
2: Make sure you have the latest GFWL update:
http://go.microsoft.com/fwlink/?LinkId=128962
If you have any issues with GFWL then MS have a lot of info & an MS staffed forum for technical help on these issues.
http://www.microsoft.com/games/en-US/Support/Pages/technicalIssuesGuide.aspx
They even have a dedicated Fallout3 tech support forum:
http://forums.gamesforwindows.com/44.aspx
3: Make sure you have the latest drivers for your graphics card.
4: Make sure your Router has an Open NAT status & UPNP enabled otherwise it will not be able to map the port forwarding required for the GFWL interface to work.
5: Make sure your firewall allows both the GFWL client through it & Fallout3.
6: Latest patch is here make sure you have it otherwise the game will not work as it should:
http://fallout.bethsoft.com/eng/downloads/updates.html
7: Some have reported that switching to window mode works perfectly when it crashes to the desktop a lot.
8: Some claim the following mod solves all their Fallout 3 issues!
http://www.fookunity.com/wiki/Main_Page
If after all of that your still having issues try the Bethesda forums:
http://www.bethsoft.com/bgsforums/index.php?act=idx
The vast majority of problems I've seen with Fallout3 on mine & other players PC's are user related to security settings on your PC which prevent Fallout3 from working properly so it freezes & crashes because of this. Many players have no issues with this now so you should be able to resolve it. Do not give up as the chances are its something someone else has already resolved on the GFWL forums.