Bios Mem Settings

I have a K75SA w/ xp1600 @ 1.58ghz (150 fsb) and 512 of pc2100 (not sure of brand got it at best buy), also I have latest cheepoman BIOS. For cooling I have a coolermaster hsf with thermal grease, no case fans. I've had a stable overclock for awhile (39-41c idle/47-48 load) so I went into the bios and tried to up the memory timing to fast instead of normal, but then it will not post. Do these memory settings in the bios make a big difference, and why won't it post on high?

Thanks

probably you changed your cas latency in the bios higher than what your memory is rated at.