Bandit: For me no....... I tried overclocking my V3 2000 from 143 to 166 and I only got a 1 fps difference (i think non at all) in half life. I'll try to take the Q3 fps soon but I doubt it'll be significant (my guess). I don't its useful to overclock right now if your games still work great. I mean, I get 45 fps in Q3 on normal mode and 43 fps on Custom 800x600 (i think) and high detail and that's more than enough for me. I was able to bear my old CL RIVA TNT for quite some time but the switch to v3 made a huge jump. If your V3 can't take the games head on then overclock it. If not it'll be fine just as it is.
Overclocking your V3 depends on your CPU. I happen to have a K-6 2 500mhz/128MB RAM around the house with a V3 2k and I tested it in Q3A a while ago on 170mhz clock speed. I only got 2 fps increase. This means that the cpu is generally limiting the speed of the video card. If you can get more than 5fps increase on your system then go ahead overclock it but if you only get 5fps or less then its not worth the trouble (and risk).