If your system is stable (i.e. no artifacts) then don't touch the voltage. An increase in voltage means a huge temp increase which will cause damage to your video card over time unless you cool it properly.
If you have artifacts while playing and you have standard cooling, take the frequency down a little bit until those artifacts are gone.
My gaming machine:
P4 2.8 GHz
2 X 512 MHz PC3200 in dual DDR mode
Intel D865PERL MOBO
integrated 6.1 MAX audio
Logitech duo MX mouse and KB
ATI RADEON 9800 PRO