First thing, never overclock with out thoroughly doing your research. It's a good way to kill your hardware, if you don't know what your doing.

I'm thinking you didn't do a driver level overclock, and instead did a bios level OC, which means it's written to your card. Throw ATi Tool back on, and set your clocks to 550mhz for the core, and 400mhz for the ram.

Also, the x1550 to my knowledge, doesn't support Overdrive. I have a Sapphire x1550 pci-e 512mb, and have yet to see or read anything stating the x1550 has overdrive support. Of coarse, knowing my luck, Sapphire punked out, and I just don`t have support for it.

The most likely reason your experiencing lag in-game, is due to heat. Overclocking without proper cooling will overheat your hardware, and bog it down. The only other reason I can think of (assuming your using agp), is that your bottlenecking at the agp bus, or you don't have enough voltage.

Do the research, you'll learn all about that, and you'll be better off in the long run.