I've never tried overclocking a Sempron (though I'm sure it's not too much different considering it's based on the T-bred core). What is your cpu core voltage set at? Even though you say that the RAM is running fine at overclocked speeds, I would definitely suggest buying some DDR-400 anyway. The less stress on the system the better. If you're concerned about cooling though, it probably isn't a good idea to adjust the voltage until you get a better heatsink and fan. Btw, may I ask what the cpu temperature averages before any form of overclocking?

The core voltage and the proper speed RAM is what helped me though. I'm running an Athlon 2500 at 400 FSB which overclocks to 3200 speeds. Core voltage at the moment is at 1.7 volts.

EDIT: After reading some reviews of the board, it seems that it isn't a favorite for overclocking though it has many nice features. Especially considering that you have voltage modifiers that are based on percentage which may not allow you to reach a stable overclock. Although I would think 1.76 volts would be enough for a stable overclock.