Graphic card to play HD x264 content

hi,
I have a "media center"ish computer, it is a P4 2.8GHz w/ 2GB of RAM and I use it to play music and video to my TV. most of the movies I download (x264 720p) are for me to watch on my much better computer on a 22" monitor but sometimes I want to play them on my SD TV, this computer can play them but it lags sometimes, the movie is watchable just sometimes it slows down (the CPU is at 100% utilization all the time). I thought about buying a cheap graphics card to play these videos, the problem is since it's a P4 it has only AGP X8 slot. so which graphic card should I buy? is it worth it investing in a 36xx if I don't plan on playing games? is it safe to go for a 2400? I understood that the ideal is a 2600 because 2400 is too weak and 2900 gets really hot (it isn't a very good case and when choosing a card you should consider the size factor) is it true?
thx,
ziv

and if I get a version which isn't XT? (I'm trying to find second hand stuff) is still ok?
nevermind, apparently almost all the versions are XT. thx

A simple google search would do; I know NVIDIA has a chart on their site that lists what each card can do (a 7800 GTX would work, for example...).

Be warned, based on my research i did for a friend, a lot of ATI AGP cards did not require x264 HD playback, and this feature was left up to the manufactuer to include. MOST 2600 cards support it, but I know not all did, so make sure whichever card you get specifically states that the feature is included.