My HTPC is a Pentium 4 3.2GHz w/ HT and 1GB RAM. It's one of these compact HPs which require low profile video cards: http://cdn.overstock.com/images/products/L10927852a.jpg which fits right on the shelves with my other electronics. My TV is a 37" 1080p Westinghouse LCD 37w3 monitor. (I sit close enough that it's a decent size)
I am very pleased with the results. The drive works as it should, and the video offloading by the 4550 seems fantastic. Running Pitch Black on HD-DVD or The Dark Knight on Blu-ray, the CPU utilization is only around 20% and it never spikes to anywhere close to max. On the HD-DVD I'm watching now at least, it seems to never spike at all. The performance is even better than graphs in reviews led me to believe. (they always used some decent C2D or even quad CPU too) The first thing I did after some basic screen calibration was watch some IMAX Dark Knight in 1080p glory. Damn that's some glorious video.
I'm also pleased with the PowerDVD 7 software that came with the LG drive. I've used older versions before. After applying a free update it told me about, it plays both formats perfectly and seems to support the latest blu-ray BD-Live stuff.
I haven't tried any games yet. The 4550 definitely isn't a gamer GPU but it will still be worlds better than the old card the thing had. I'll probably try Psychonauts with a controller or something.