There's no question who rules the prebuilt SFF system market. Shuttle was the first true innovator, and haven't rested since taking over. If you're looking to build a small form factor system, one of Shuttle's XPCs is probably the best place to start looking.
But they have one major disadvantage; due to the proprietary format of the motherboards they use, they are not upgradeable. Even if your old Socket A G chassis is alive and kicking, there is no way to fit dual core Athlon in there.
In that case, the next best thing is a small form factor case, which uses an open mATX format. The Sugo SG01 from Silverstone is what we'll be looking at here. Although it's bigger than the Shuttle SD31P, it has some distinct advantages.