UT runs perfectly on my Elsa Gladiac (gf2 gts), but I have had some instability problems when oc\'ing passed 1Ghz. Nothing major though, 900Mhz is more than enough. The engine is admittedly pretty basic.
Q3a just blows UT out of the water in visuals (1600x1200... yum) whereas UT has better map designs and an overall more "fun feeling". I guess that has something to do with the kickass announcer, variety of games, and multi-firing rocket launcher.