Rapide23, I just installed a 4 GB Transcend 133x CF card in my H320 yesterday, and it works as advertised (meaning Rockbox works perfectly, but the original firmware can't read from the card).
As the CFModGuide that petur linked above is a bit cluttered, note that the first thing you'll need to do is download the H300.zip file from that page (which is a modified version of the 1.29J firmware) and upgrade your iRiver with it (do this while you have the hard drive in it - and both the hard drive and CF work with it). I personally transferred the .rockbox files onto the CF card before installing it, but I believe you can copy those files to the CF card after installing it in your iRiver and plugging in the USB cable (while it's off so it goes into bootloader USB mode).
Also, I personally bent over the pins for the jumper, ground down part of the plastic on the adapter, and cut a little notch in the rubber that sits between the battery and the hard drive so everything would fit better (other people may have done as little as bending over the jumper, but I don't want to risk damaging my battery by squeezing everything in - as it's a tight fit; though it's less than a millimeter too thick after also removing the piece of rubber that sits between the bottom of the hard drive and the circuit board). The pictures below show most of this. (I ground the plastic on the adapter at an angle as it sits at an angle in the iRiver, and the little rectangle in the second picture is where I cut a notch out of the rubber.)
Another thing is that opening up an H300 series iRiver isn't as easy as the H100 series. If you haven't already done this,
this page details how to remove the back cover (the rest of the instructions aren't important). I personally started on the top and bottom (which are easier to pry apart than the sides) then moved to the sides using a knife that's beveled on one side and flat on the other (plus a credit card once there was room for it). I had the beveled side facing the back of the iRiver so it would push the tabs back (so they could be unhooked without breaking them).