I don't think you'll have any luck on that one. AE is quite stubborn in that regard and encoder matrices, stream multiplexers etc. are hard-coded. You probably will have to stick with using external tools for the final conversion.
You'll always get a better result rendering to a lossless or nearly lossless codec out of AE first and then using third party software to render your H264. AE cannot do multi pass encoding and that's critical for the best compression.
I'm not saying I don't believe it, but it just seems so bizzare they would actually have a "PSP Preset" but not make it the correct resolution. I thought maybe it was a CS3 thing, but have confirmed that CS4 requires the same reduced resolution output.
Thanks, Rick, but th problem is not with the lack of multi-pass encoding ... it's the fact that the PSP preset is forcing you to stretch your footage to a lower resolution. I can live with the single-pass (it's for a PSP after all, LOL), it's just the reduced-res and required stretching that is a problem.
then you can try to go to the h264 settings, and creates you own settings with the correct resolution (and make sure all the other parameters match the psp preset).
SHould be working.