I've done some testing on an old laptop & NEC CRT monitor I just retired. It was only Win98SE (no kidding) and the NEC was 15 years old. It had very little power saving in sleep as it basically only shut off the harddrive. Maybe only 1W. For the NEC the sleep/standby was around 8-10W.
One thing I noticed with the NEC was how different color backgrounds made a difference. With my normal blue desktop both drew about 80W, but when I changed to a white background in stayed about 86W. I tested with difference screen savers and noticed the same behavoir. I will be doing similar testing on my Sammy LCD monitor in the next few weeks and I will report my results here.
Getting back to HTPC, my current machine which I use as an all-in-one is a Athlon64X2 (6000+ 3.0 GHz (89W)) with Vista premium. I hope to test it this weekend, but I use S3 sleep and expect it to only draw 10-20W in sleep. This does not include the LCD but I usually shut it off every night anyway. I have to say that the S3 sleep works great. The remote and wireless keyboard and mouse receivers still get power and are able to wake up the PC within 5 seconds. Over 4 weeks I only had trouble waking up the machine with the remote/keyboard/mouse only twice, and when that occurs all you have to do is press the main power button. For you with XP MCE 2005 it is possible to set up S3 sleep using a tool called dumppo, but it depends on your hardware and bios.