I've bought 2 shuttle mini PCs (SB51G and SS51G). I would recomend either, although the SB51G (based on the intel 845GE chipset) offers more features that aren't really listed in the specs that can be useful in dialy use.
In my SB51G I put a Celeron Northwood 2.0GHZ and 2 256MB sticks of crucial PC2100. The macine runs Mandrake 9.1 and is currently my web/game server.
The SS51G I purchased for a friend. It also has a 2.0GHz Celeron, but has a Radeon9000 w/ DVI since the computer also uses a flat panel (Samsung 172T). This machine uses a single 512MB stick of Kinsgton PC2700.
On mine, I've tweaked the cooling to be nearly silent, but on my friend's I left the stock cooling so as not to void the warranty.. the PSU fan on that computer is a little rattly (that is my only complaint about the shuttles.. their use of 40mm PSU fans)
However, both machines run flawlessly and I am 100% satisfied.