<< Would RTCW be playable on my rig listed below at say 1024x768? What kind of fps could I expect?
Also, what single upgrade would be recommended to give a decent boost? (single component like CPU, Video Card, etc) >>
With your current rig, expect FPS less than 40 constantly at 10x7 in 16 and 32 bit mode. The game is a pig, much like the rest of any Quake3 games. If you wanted to get much smoother and faster, get a newer Nvidia based card -- that'll give you the most noticeable boost.
vash