For your needs, the new Intel
For your needs, the new Intel Sandy Bridge Pentium parts may be a good solution. The G620T paired with an H61 or H67-based Intel-branded motherboard should give some great results when it comes to power consumption and heat (or lack thereof) as well as taking care of the other tasks you mentioned. The Intel board also will have an Intel NIC which is preferable. If you get the H67-based Intel board, it will have the Consumer Infrared (CIR) header for connecting an internal IR receiver.
You should only need a discrete GPU for the Intel parts if you need 24p output and you aren’t willing to disable UAC. AMD boards will probably have one of the 42xx IGP parts which are not as feature-complete as Intel IPG. Specifically lacking with AMD are support for 3D material, advanced de-interlacing and audio bitstreaming capabilities.
The pico should be adequate if all you are powering is the motherboard, processor and a couple of drives. If you do want to go with an ATX supply, most decent supplies should be quiet when you are just using them for HTPC duty since they are nowhere close to a load where the fan would have to spin faster. The pico will be more energy-efficient compared to ATX supply.