Well you could go for Android TV Box although you need to be careful with chipsets since some running XBMC don't have native or limited hardware decoding and thus require software render.
I believe the Rockchip RK3066 (dual core) and RK3188 (quad core), All Winner A20 (dual core) all appear to have XBMC support but you'll need to check the XBMC forums.
I saw this on eBay the other day http://www.ebay.co.uk/itm/AndroidBoxTVT ... 1240845651
for 50 quid it's not bad considering what you're getting: AV + HDMI + TOSLINK, Wifi, Ethernet LAN, USB hub and remote control.
If you still want Linux then you're probably better off getting a Beagle Board - http://beagleboard.org/