You can use something like this:
http://www.amazon.com/Composite-S-Video-Converter-Scaler-Adapter/dp/B005CV4ZH6/ref=pd_cp_e_3There are a lot of them on the market that look very similar to that, and they're all rebranded Lenkeng
devices. The original website is here:
http://www.lenkeng.net/These Lenkeng scalers have a bit better quality than your standard HDTV scaler, but still look worse than a CRT. They get the job done though.
If you want the best quality, you need to invest in a real upscaler, like the Micomsoft XRGB series. They're pricey though, and not really worth it if you're just feeding composite into it. If you get your system RGB modded though, that's a different story.