Basicly in OpenGL you have 1.5 x 1.5(2x type mode) and then the two 2x2 modes. With a MX you could expect about 40fps average(timedemo 001) in 16bit in Q3 using the 1.5 x 1.5 setting. Your basicly looking at the same thing as 1536 x 1152, inbetween 1280 x 1024 & 1600 x 1200.
And yeah it would look pretty good at that setting. Try disabling hardware gamma if your going to play in 16bit. Put set r_ignorehwgamma "1" & set r_gamma "1.5" in your autoexec.cfg. Adjust the r-gamma setting to your liking, higher is brighter. That will greatly reduce the appearance dithering, and the FSAA will help also with a bit more color info(20 bit or so).
As for D3D, I more often then not use FSAA on my Geforce DDR. Mostly at the moment because I am on a 14" because my main monitor just fried. :| Its a trade off at higher resolutions, but it is often playable in a lot of D3D games. I usually set it to the max and try and see if its playable.
My current stance is that I get it to run at 1024 x 768 first then try out AA options. I find that to be the best trade-off depending on your monitor.
BTW, a GTS should get around ~70fps at that setting, and an Ultra somewhere around 90+.