I guess the answer is "yes" but, will UT2k4 work with the great Alien Swarm mod? I ask cos it's more like a Total Conversion than a mod, so better safe than sorry :)
Thanks in advance!!

Yes, it'll work with all mods/total conversions. I don't know why it wouldn't as it installs itself in a separate folder in the UT2004/ folder.
Alien Swarm rocks ;>

Yeah, the one for UT3 is quite funneh, but it needs weapons imo :D

If alien swarm will work, will the Killing Floor? Sry if this is a stupid question but that would be the reason I buy UT 2004.

Of course yes. Killing Floor is teh awesome. Let's be clear, there isn't any tweaks the GOG team will apply to the game. ANY. It runs flawlessly on every system... It just needs a valid cd-key to play online, and the latest patch 3369 removes the DRM (it includes an official no-cd). So the mod compatibility will be full.

well then will GOG be supplying us with cd keys for online play? Again I apologize if this is a stupid question but I haven't purchased any multiplayer games yet from this site.

Wow, I had actually forgotten about Alien Swarm! What a blast...and yet another reason to pick up UT2004.