Over drive is a system ATI incorporated into the xt series to overclock the card when it's cool (which also means you should have temperature monitoring abilities, I will have to check on that). However I don't believe it does much overclocking. ~30 MHZ I believe. You would be better off doing it yourself. Programs like Radlinker, Rage3dtweak, and power strip should get you started.
My process for overclocking video cards is fairly strenuous but produces very nice overclocks.
1) Wait for a pretty hot day in your room so that your computer doesn't keep crashing in the summer :-P (If you oc in the winter)
2) Then slowly increase the core and memory speeds. In general, I like to keep the same kind of ratio as the stock settings. Eg:
Your card is 500 core / 300 memory ... so In general I would keep a ratio of 5/3rds .
3) Increase slowly and run a strenuous benchmark. I increase in 2-3 increments until I start seeing artifacts in the program I run (3dMark2003 + Aquamark). once you start seeing artifacts go back down in 1 mhz increments and once you finally think you've reached a part where there are no artifacts. Run a burn in test for about 2 hours and keep an eye on it.
4) Make your final tweaks and this yields your max overclock for that room temperature. They overclock varies with case temperature which varies with room temperature obviosly. Which is why I do it in summer (Unless you want to run two different speeds Winter and Summer
However, if you have temperature monitoring abilities in your xt, check out the temperature and once you start seeing the artifacts look at the temperature then try and clock down til it runs about 5 degrees cooler