You might want to try and go into the Advanced Audio settings and disable any assiotion to 3d hardware support. All that does is causes the sound to become 3d if you have a 4 speaker setup. In your case I would imagen that correct sound would be more important then 3d sound
Also with your system you shouldn't be having any issues with RAM unless its gone bad. I played UT1 on my old P3 600 /w 256mb and it ran just fine.
Oh one more thing you could try, see if Environmental effects are enabled for UT, if they are turn them off. sometimes they like to cause a serious echo in HL so I donno what it could do in UT.
Gluck