They will be fine in the older D3 based games at medium (12x9/12x10) resolutions, as long as you don't mind the frameskipping issue with that engine. ETQW is supposed to be somewhat more demanding, but should be okay if you turn down some settings.
UT3 will probably make them struggle unless you turn down everything close to the minimum. However, this is not such a bad thing in a multiplayer game. I'll have a new card by the time UT3 is out (and am highly looking forward to that game), but I often do that in multiplayer games to keep the framerate sky high and don't pay much attention to graphics when playing online anyway (if anything, they're actually distracting).
If you don't like that idea, I think the cheapest card that will give you a reasonable amount of future-proofing is the X1950XT at $170.