I bought an HP MicroServer N40L (AMD Turion II Neo N40L dual core) two years back, and added in a low-profile AMD Radeon 6450. The combination is powerful enough to host a media server, play some older games and stream more modern Steam games from my Intel desktop, but low-power and cheap enough to make me very happy with my purchase.

The new generation MicroServers are Intel Celery pieces - more powerful but far more expensive (although I can't say for sure whether the price tripling involves other factors).