Full Res Advice

So, here's the story. I'm running 3.40 OE-A (or was it 3.50 OE? Can't remember, too lazy to check, either way: it's hacked), and I have a batch of H.264 (baseline) clips at 500 KbPS and 480x336 (4:3). From what I've read, legit firmware at 3.40 and higher should be able to play H.264 (mainline with CABAC) at resolutions as high as 720x480, but there's supposed to be some sort of trick to get it to work. Is there any advice people can give me regarding this? I'd prefer just to be able to tweak my MPEG-4 header since I know the video data is compatible, and I'd rather not re-encode the video (Hell, if I was going to do that, I might as well just go for 352x272 or whatever).