Not much is needed just for streaming movies. I went the same route but I also built it as a gaming system to replace my X-box as well.
I'd suggest looking at an E-350/E-450 system or a Celeron 847 based system. Either one would cost about $80 for the Motherboard/CPU. If you want WiFi, most have PCI slots or you could just go a plug n play USB route.
I'd suggest getting a decent case for it that preferably is silent, the reason I mention it is that I have tried several ITX cases for HTPC use and my biggest complaint is noise from the fan or the power supply. I found the Antec ISK 300-150 to be a pretty good case, and it can be had for about $80, it also has a low profile PCI slot in case you want to upgrade later on. It will only take 2.5 in drives however. I also have used the Thermaltake Element Q, that one isn't bad either but the PSU has a loud fan. I switched mine to a Diablotek that was a bit quieter, it also supports regular 5.25 drives and 3.5in drives. The best cheap ITX case I've tried so far was a Cooler Master Elite 120, it was very quiet, had room for several HDD's and a full 5.25 drive bay, it also supports regular size power supplies but it's pretty tall for an ITX HTPC and very long.
Might check out a Zotac ZBox or something along those lines. Roku, X-Box 360 and PS3 also make great HTPC's!