I'm just curious if there is any official statement from GOG on running the games on Vista x64, or XP x64 for that matter. I got a new quad-core box with 8 gigs RAM recently, and so I installed Vista Ultimate x64, so I could actually use all the RAM. But I've found since upgrading that legacy application support, especially for older games, is poor. So does GOG test all their games on 64-bit Windows?

I don't know, but I too have a quadcore, the only reason I haven't gone for x64 is driver compatibility and software compatibility, there's really only one application that I know that runs in x64 that I would enjoy (steam) the others I don't know and don't care, back when I was in the Vista public beta, they didn't have support for my sound drivers, so I said fark it and left it alone.
you know man, maybe being on x64 isn't worth it. maybe it is, I don't know your situation, but personally, I'll stick with x32 because that is what all the other programs and features I need, use.

I'm running Vista x64 Ultimate, and have had no troubles with the two games I've tried (Kingpin and Sacrifice)>
Pretty sure a GoG staffer posted saying that all games are tested to work on both XP and Vista 64 bit variants.

I have Vista 64 (C2D +4GBRAM) and MDk2 runs great, runs widescreen and uses my gamepad with no problems. The other game I bought was the JA2 expansion and it starts okay, but I've yet to play it.
As for Vista 64, it's the only way to get support for 4GB and above. Most people might not need that now, but... The drivers are working well now. NVidia finally added custom timings for example. The only issues I have that I wouldn't have on Vista 32:
DRM schemes like Gametap's windows games and the DLC from Oblivion that don't work well with it. The Oblivion DLC can just be installed on an XP machine (Real or virtual) and copied over into your data folder. Bethesda may have fixed this, but the last I checked, GameTap had no intention of even trying to support 64-bit. Their DOS and other games still run fine because they don't use the same DRM as the Windows games.
16-bit programs Will Not Run. Most 16 bit programs I can run in DOSBOX, or Virtual PC since they aren't too demanding, but there are some cases like Jedi Knight 1 where the installer is 16-bit while the game is 32-bit. Just an internet search will give you a way to do a manual install though.

8GB of ram is kind of overkill for videogames. What are you doing with that box?

Maybe he's a server.
I've bought Fallout: Tactics and it worked flawless in my machine. I have a E8500 with 4Gb RAM using Vista x64.

I'm a developer and the extra RAM helps running multiple instances of Visual Studio and IIS/debugger/etc.
Since making the original post, I've played Freespace 2 for about an hour, worked flawlessly. I just installed Fallout 2 and the intro video had some visual corruption, going to play it a little and see how it goes. But glad to hear the official word on this, GOG is doing a great thing here and I hope they can be successful. w00t