I really like the Shuttle SG31G2B. It is very complete, has onboard video (GMA X3100), but handles modern Core2Duos/Quads like the 3ghz E8400 or a Q9400, and has space for a decent PCI-E vidcard. I think even a 8800GT fits in, but I run it with a 7600GT. And it is way cheaper than the gaming-shuttles.
An entry-level MacMini is great too (as PC as well) if you want a basic 2ghz core2duo machine with everything inside. Because it has BT, you even can use a remote keyboard with ease.
If speed does not matter (oldstyle maming only), than go for a first generation EEE-notebook. They are good on the external displays, and pretty complete machines for 250$ or so.