My G5 is still my main machine. There are really only three reliable upgrade places for my money (there are, of course, hundreds in existence):
OWC/Macsales: If you're lazy and don't want to do research, OWC is the place to buy anything for older/legacy macs. All their stuff is guaranteed to be compatible, but the price tag will be higher than elsewhere. I sometimes use them as a resource for compatibility, but the last thing I bought from them was a buggy dual G4 upgrade that they tested and said was fine, even though it wasn't (though the people were very nice and as helpful as they could have been). Oh, and the price dropped $40 about two days after I bought it, but that was just bad luck.
www.macsales.com
Ramseeker--for ram there's no better site I've found; they have drop down menus to choose your model/RAM type, then they list about 10 or so sellers and their size/prices. Be careful when rushing through, though, as sometimes the seller's price will have changed; a friend of mine overpaid for RAM because he didn't double check until after he'd already purchased.
www.ramseeker.com
NewEgg--everyone should know NE by now. I've been buying from them for god knows how long, probably about a decade. Best costumer service EVER, great deals, etc. but you really need to do research for Mac compatibility, especially with older machines.
www.newegg.com
As for a mailing list or newsgroup, posting here is always good, and there is a G5 mailing list at lowendmac.com, but I'm not on it so I can't attest to how good it is...