HDMI video cards DO exist, but they are more then likely pricey.
MSI's NX7600GT - is an HDMI video card that starts around $190 or so. I also recommend.
Check out these links for some ideas about building a Multimedia PC.
http://www.pcmag.com/article2/0,1759,1602268,00.asp
http://www.youtube.com/watch?v=ldJNp6wdfqI
http://electronicdesign.com/Articles...eID=13330&pg=9
http://www.electronichouse.com/artic...i_media_pc/C43
I would use newegg.com to for the hardware. I've always had good luck. Start pricing the parts you want to use. I would suggest getting 2GB of memory if you decide to go with Vista, just to be on the safe side.
USB should be on the mother board, that shouldn't be an issue.
Wireless I recommend Linksys. I've always had good luck with their products.
Harddrive, the bigger the better.
CD/DVD RW are easy to find at a reasonable price.
Power Supply 400W would be minimum. I would figure out what video card you are going to run because some of the newer ones require higher power Power Supplies.
Motherboard and Processor, I would get a Dual core at minimum..