Yeah you're kinda stuck for AGP, bu if you can avoid the GF7300 and get at least an HD2400/GF8500 (unfortunately no AGP GF8 right now) that would be the way to go, anything less means you have poor HD support, and a GF7300 means you have less HD power than the X1550, so between those two go with the X1550. The GF7600 (and really a GT) is the lowest GF7 series for HD support, and the X1550 is the lowest for the X1K series (the X1300/1400 are ok and maybe better than the GF7300, but both are too weak for any real heavy lifting), and all the GF7 and X1K get destroyed in HD playback by the HD2K series. So IMO the HD2400 is the best option for any new HTPC.
Anything involving serious decompression help you should keep strictly to the HD2400 series (I'd otherwise also recommend the GF8 series but no AGP let alone low profile AGP for that).
You have your work cut out for you IMO, and maybe if they bring an HD3400 to the AGP market that might be your best option, but that would mean waiting a long time.