there's really nothing avaliable yet for aftermarket parts on them, probably be another month or two before we start seeing any sort of upgrades for it.
I just picked mine up yesterday and I love it so far, the only downsides on it I would say is the rear visability isn't great and the mirrors are too small compared to my last vehicle, my last one was a dakota though so it had huge mirrors and a lot of rear visability compared to any car, oh and the card rides so nice that you don't notice you're speeding, I looked down at my speedo and noticed that I was doing 60 in a 30 zone on a winding road, and it was more stable at that speed than the dakota was at 30, the perfomance on it is very nice, even with it not even broken in yet it's a lot faster than my dakota R/T which had some intake and exaust work done.
here's a good forum with info on the magnum and 300:
http://www.dodgemagnum.net/phpBB2/index.php