For Intel, take the ION for its HD playback graphic capabilities.
For AMD, an Athlon II X2 2.7GHz with 785G chipset motherboard is more than enough.
Regarding the RAM, since you're aiming an HTPC, single channel or dual channel doesn't really matter. But for performance sake, go for the dual channel (2 sticks of RAM)
Get an 80+ PSU with active PFC. a 300W - 400W would do just fine. Don't risk your system by using low powered and low reliability PSU.