Will this mess up my graphics card?

All you can do is try as every card is differant. If ATI overdrive wont work it may be that its to much of an overclock & the card cant handel it. Try increasing the core in 10MHz steps then do the same with the memory. Watch the temp of the card & when it starts getting hot back it off 10MHz. You dont get much of a performance gain by a small overclock & if you plan on a large overclock you will need to buy an aftermarket GPU cooler as the stock 1 wont be able to cool it & you will burn up the card. I dont overclock anything on my PC & if I get a new game that wont play smooth with no lag I buy a new video card. I have an ATI HD 3870 X2 1GB GDDR3 card & can play any game I have bought out at this time