Hi, I havn't posted in a while but have been reading posts with interest.
This question caught my eye and I think I can add some things... ;\
http://www.jaquays.com/ - the Game Art Resources section of Paul Jaquays (ex id Software) site has a lot of good links that he compiled originally in the forum at
http://www.map-center.com/ http://www.planetquake.com/hfx/ - [HFX] by Yves Allaire, another very talented artist who also produced a number of community texture packs for Q3A, all free to use for non-commercial etc. (unfortunately he's hosted on PQ so there's a lot of annoying adverts from GameSpy)
I know quite a lot more but don't want to clutter the post with a whole list of links. [confused]
It may also be worthwhile reading the Shader specifications for Q3A or one of the more modern games if you havn't already because some of the funky bits of the rendered textures are coded modifications to the textures.
One last thing. Quite a significant number of the original id textures for Q3A used rendered 3D models as a starting point and this is now common practice.
I'm sorry I don't know any of the specifications for textures for UT2k3 because my PC isn't strong enough so I havn't done any game textures for a while. :{

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I hope I can be of help and am not telling you things you know already [innocent]