The Acer Aspire Revo 1600 is by far my favorite Ion based machine that can be used as an HTPC, in fact I own this machine and even started a website on how to set it up as a perfect HTPC, http://www.RevoHTPC.com. This is one of the lower powered machines as it only features an Atom 230 which is a single core Atom chip. It also come pre-loaded with Windows XP and only has 1 GB of Ram. However if you already have a copy of Windows 7 or want to run Linux on this and don’t mind shelling out an extra $25 for another GB or RAM then this might be a perfect HTPC for you. Here is the stat sheet:

Processor Atom 230 (Single Core)
Memory 1 GB (4GB Max)
Hard Drive 160GB 5400RPM SATA
Video Ports 1 VGA, 1 HDMI
Other Ports 5 USB, 1 eSata, 2 jack Audio Ports
Operating System Windows XP Home
Wifi None
Optical Drive None
Included Accessories Mouse, Keyboard

I do absolutely love this machine and have used it for many months now. The greatest part is that it retails for only $200. It is available on Amazon and Newegg.
Amazon:

Newegg: