I think that the iMac Core Duo would handle HD fine. Apple says the system requirements for editing HD with iMovie are "High-definition video requires 1GHz PowerPC G4 or faster or Intel Core Duo processor; and 512MB of RAM", which your iMac would easily qualify for. But that's for iMovie. If you are planning on using Final Cut Studio, the iMac might not handle that as well and I would probably recommend a Power Mac. I'm not completely sure on this, because I think that the iMac would be able to handle Final Cut Studio based on the system requirements Apple gives, but it would make it a lot faster and easier on you to use a Power Mac for Final Cut Studio. But, if you decide on a Power Mac, it would probably be better to wait until Apple comes out with an Intel version, even most of the apps are universal, that would be what I would do if I was lucky enough to get a new computer, because I would be afraid of being out-of-date, and it is likely that the Intel versions would be faster (based on the trend of all the other Intel-converts being faster) and it could possibly be less expensive. But in general, if you are just starting on learning film-making and editing, I would stick with iMovie and the iMac. Final Cut is a little bit overwhelming and has a very steep learning curve compared to iMovie.
As for RAM, I just recently bought some for my eMac from :
http://www.newegg.com/
This website has great prices and fast shipping. At Newegg.com, I would recommend looking for
crucial memory, which is always very reliable. But the RAM at ramjet looks good too, whichever you prefer. If you get it at newegg, you'd have to be very careful to get the right type, as there is a lot of specifications you have to look up and match. It took me at least a week to decide on the correct RAM for my eMac.
Just out of curiosity, how are you going to get HD footage? From what I read, I didn't see that the Panasonic PV-GS65 camcorder was able to shoot in HD.
Good luck!