My 2 cents, get the 400. If not for the speed, get it for the additional RAM/ROM. Once you find all the cool software out there that you'll want to use, your storage will start filling up. I don't think the money you'll save on getting the 300 is worth it. I would recommend watching a site like
http://www.bensbargains.net or
http://www.techbargains.com and just wait until you can get the 400 for around $250+. That's how much I paid for mine. It was a great deal and it's totally worth it. Also, since you mentioned the 600mhz, just FYI, I've successfully overclocked mine to 590mhz. :) I don't run it at that, I run ~500mhz, but still, it's possible.
hypnotoad