I just built a similar htpc on a little tighter budget, but a lot of the components are the same. Posting from it right now..
CPU - Sempron 140 unlocked to dual core Athlon II 4400e (45nm) OE cooler
HDD - Segate 1.5tb sata 7200
Case - Apex DM-387 MATX
Mobo - Gigabyte GA-MA785GM-US2H
Ram - 2x1GB Gskil DDr2 PC6400
GPU - Onboard 4200HD
Optical - DVD-RW Liteon sata
TV Tuner - HD 650 ATI PCI-E
Wifi - PCI Wireless N (Dont remember the brand)
Did it on a little less than $300 total, pretty darn cheap if you ask me..
For mouse and keyboard I am using a normal ps2 keyboard and usb mouse for setup, and my Iphone with the Air Mouse app for HTPC functions, it works pretty well.
As for performance, I am pleased with the efficency I dont have a killawat meter for it, but the heat it generates is minimal, and plenty of power for Hulu Desktop, running windows 7. and flash 10.1 beta 2. Just getting all of the codecs setup and media center setup with the TV tuner, but so far so good. With the quad core you have more processing power, but for me the dual is enough even with hulu 720p.
DVD playback is great, replaced the case fan with a ball bering unit and it is very quiet, and so far no hickups as far as reliablity, I have not built a computer in 5-6 years and it went smoother than I remember.
You should be good to go with that setup.