I've been using ATI Tool Beta 13/14 since Feb. with great results. Here is what I've done (and a ton of other people too):
1) Install ATI Tool
2) Disable the ATI Hotkey and ATI Smart services and than reboot.
3) Set voltages to where you feel the limits are or where you want to start testing. I'd recommend 1.475-1.5V as the max on the core with stock cooling unless you have very good temps (<75-80C at load). My memory responds well to increased voltage but I have it cooled well. I use 2.2V to hit 875MHz and 2.15V for 850MHz. I probably wouldn't go over 2.1V on stock cooling. The last value (VDDQ??) I would give 1.5V with no worries. Others have gone well above that but with stock cooling I wouldn't risk it.
4) Go back to the gpu/mem clock screen and start experimenting. I usually test the memory and gpu separately. Just increase clock speeds by 10MHz and than run the artifact scanner for 5 minutes. It is actually pretty accurate and will get you close to your max clocks. Sometimes it won't register an error but you can see little artifacts pop up on the spinning cube of carpet.
5) Once I've hit the max clocks on the mem and then on the gpu, I'll test them together. It usually means I have to lower both down a bit. After I've found the max by letting the artifact scanner run for 15-20 minutes with no errors, I'll count that as my max. 90% of tha games I've played will run just fine if the artifact scanner didn't pick anything up. A few tax the card more than ATI Tool so you'll have to experiment.
6) Make a profile by saving the card clocks and than make sure you go back to the voltage menu and save the voltage settings to the profile you just created. Otherwise the clocks will be saved but your voltage will remain at stock when you load a profile.
7) Now when you game, just open ATI Tool, load your profile, and you're ready to go.
I know that was pretty simplified for most of you but I really don't see any problem with ATI Tool as it's worked very well for me using CAT 6.2, 6.3, 6.4, and 6.6.
Here are two great threads over at XS that have a lot more info if you're interested.
Overclock Voltage Guide
Eva's Info