Speaking from my experience with the GS-500, I can say the video quality is excellent. Many consider it to be near broadcast quality. I am not sure about the differences between your 300 and the 500, vis a vis the CCD size, so I can't say how the two compare. If you want to pick far smarter Panasonic knowledgeable brains than mine, go to this Panasonic user's website. Amateur and professional videographers frequent this site.
http://www.pana3ccduser.com/forumdisplay.php?s=8b722d3c98c5012437a68ecef324cdfb& daysprune=&f=6
Of course, a HD cam will get higher quality video, but from what I have read here, there are other editing/DVD burning obstacles to overcome with HD. You would probably want to be using Final Cut software to work efficiently with that. I'd say give your 300 a chance to see what it can do first. Unless you have all the expensive equipment needed to produce, edit and playback HD content, you will probably see little difference in the final result. Others more knowledgeable about HD might have comments here.