Low power "upgrade" 24/7 htpc / media server

I'm looking at changing my homebuilt system around to create a lower power htpc. Right now my system works great, I primarily run XBMC, watching 1080p mkv's and SD tv shows. I don't need speed, just as long as the movies are accelerated so that they play smoothly. This is what

right now i'm running two separate systems, one 24/7, an old laptop, Asus S5N which is:
Pentium M 1.6ghz
768mb ram
Windows XP
external USB hard drives (a 2.5" 160gb d/l drive and 1.5tb seagate)

this system primarily runs sabnzbd+ and sickbeard, and serves via smb all my tb shows.

I'm sure it's relatively low power, but it's no Atom processor.

Then my HTPC is:
GIGABYTE GA-MA785GM-US2H
AMD Phenom II X2 550
Geforce 240gt
OCZ ModXStream Pro OCZ600MXSP power supply
4 SATA hard drives, 1x 320gb 7200rpm (OS), 1x 1.5tb seagate low power, 1x 500gb 5400rpm, 1x 2tb seagate lower power
Hauppage 1600 pci tv tuner (which never gets used, besides the remote)
2gb ddr2 pc6400 ram

I have that computer sleep after 20 min of inactivity. I'd like to consolidate to one system. Was thinking a few things.

Pick up an ion netbook as the htpc/24/7 server & something like this:
http://www.newegg.com/Product/Prod [...] -_-Product
(4 bay sata/usb enclosure) ($110 + $350 for netbook, sell old system)

pick up an ion motherboard and replace in the current system (about $179, sell old motherboard/processor)

pick up a AMD Athlon II x2 250u (25w) or AMD Athlon II 160u (20w, but single core) and keep everything the same.

any suggestions? thoughts or estimates on the total wattage at idle, and playing a 1080p mkv of these different configs?