I just finished overclocking my cpu to
558mhz from 450mhz and i also put in a tnt2 m64 card and now i get a grey screen everytime i go to play Q3 
my cpu temp is running the as it was before
so i dunno?
do u have a amd k6 2 or 3 ?
if so do u have via appollo mvp3 chipset based mother board?
if so download via 4 in 1 patch from www.viatech.com as well as check for bios upgrades that may solve the problem
clock your processer back to normal
get latest drivers from your video card
and the minigl driver if it is a 3dfx based card
if nvidia card make sure u installed latest
direct x first before nstalling drivers
set display properties to default settings
try running quake2 (if u have it) in software
mode if it runs then change the renderer
to open gl in the video options if q2 runs in software u will no it is not the game and q3 should run if q2 runs in open gl
if u can run d3d then trying running 3d mark in continuous loop to see if overclocking
effected it or not.