System 7.6.1 is what I run on my 520c, and it runs well. Web browsers won't run fast on these 25/33 MHz Macs, but it will help if you run a more modern web browser such as Netscape 4.0.8 or iCab 2.9.9 - which you can download here http://main.system7today.com/otherbrowsers.html

I don't think this forum software is very friendly to old web browsers - I tried posting this message 4 times on my 520c, but wasn't successful.