XBMC and x264

I've started playing with h.264 and encoding with x264 with megui and I like the results. I am playing it through a modded x-box runnng somewhat the latest XBMC (1/07). I've finally settled on the following with a resolution of 640 x XXX for playback without any dropped frames: --pass 2 --bitrate 976 --stats ".stats" --no-fast-pskip --bframes 1 --b-rdo --bime --direct auto --nf --no-cabac --subme 7 --analyse all --8x8dct --qpmin 18 --vbv-maxrate 25000 --qcomp 0.3 --me umh --threads auto --thread-input --progress --no-psnr --no-ssim --output "" "" My question is, what other options can I play with that will improve quality but not be more CPU intensive for playback. Will setting "number of b-frames" to 2 be harder on the playback CPU. I currently have it set to 1. When I set it to 2 b-frames, B-pyramids open up. What is this, will it help quality and most importantly, will my x-box be able to play it? Also, I don't have my x-box set to the hi-def mode, it's in standard def right now. Any suggestions appreciated. Thanks.

XBMC uses libavcodec, so these should work fine.

Thanks for the replies. It seems the only way to get --trellis 2 is to enable CABAC, and I understand CABAC is a CPU killer during playback Following is my new setting and no skips yet :). --pass 2 --bitrate 896 --stats ".stats" --ref 3 --mixed-refs --no-fast-pskip --bframes 2 --b-pyramid --b-rdo --bime --direct auto --nf --no-cabac --subme 7 --analyse all --8x8dct --qpmin 18 --vbv-maxrate 25000 --qcomp 0.3 --me umh --threads auto --thread-input --progress --no-psnr --no-ssim --output "" "" How about "No Dct Decimation" and "Weighted B-Prediction"? What does these do and will enabling them incur a playback penalty? Thanks.

FYI - the following x264 options works well (no skips) with XBMC at a resolution of 640 x XXX. --pass 2 --bitrate 557 --stats ".stats" --ref 5 --mixed-refs --no-fast-pskip --bframes 2 --b-pyramid --b-rdo --bime --weightb --direct auto --nf --no-cabac --subme 7 --analyse all --8x8dct --qpmin 18 --vbv-maxrate 10000 --qcomp 0.3 --me umh --threads auto --thread-input --progress --no-psnr --no-ssim --output "" "" --trellis 1