I have recently replaced a WDTV Live with an Asus 1501P nettop, it has the following spec:
1.8 GHz Intel Atom D525 (dual-core)
NVidia Ion2 graphics
2GB DDR2 RAM
250GB Hard Drive
Slot-loading DVD burner
Two USB 3 ports + some USB 2 ports
Windows 7 Home Premium
I am very impressed with it having previously tried an ASRock Ion 330 last year which was too noisy (imho) for HTPC use in my bedroom.
I've installed the latest beta of XBMC on Windows 7 and it is all very slick, much smoother than I expected. I was also pleasantly surprised to find that the Asus comes with a media centre remote and has a built-in sensor. It runs fairly quietly with only a slight woosh of air that I can definitely live with as you can't hear it when playing media.
The Asus cost around £372 including delivery and is a very well built machine for the money.
This was a far cheaper option than building a machine from scratch, although I have built a beefier HTPC for my living room (OrigenAE M10 case, Intel Core i3-530 CPU, 4GB DDR3, Gigabyte H55-USB3 motherboard, circa £600 a few months ago excluding an operating system).