Not sure why everyone says it will hurt your OC. Just run a ram divider and keep the ram at 200mhz. Then you can bump up the FSB all you need. Only thing you'll be missing out on is faster ram, which doesn't have a huge effect on system performance anyway. (Compared to other things)
The Sempron's are suppose to be pretty good clockers. Not sure if you can hit 50% though, that'd be a pretty good clock. Someone that has worked with one can probably tell you better.