I just built something real similar to that, if you want to take a look. I'm going to go ahead and add it to my sig now, but if you want to come over some time to see it let me know.
Basically...
Soltek 75FRN2 ($30 refurbished from Newegg)
WD 800JD ($40 refurbished from Newegg)
2x256 Kingston CH-5 ($60 from classifieds)
Antec 3700BQE w/ 350w PSU (traded for something in the classifieds)
Radeon 9550 (part of two trades + upgrade deal)
Old Floppy
Whatever you want for an optical drive
It turned out pretty nice and fairly inexpensive. It helped that I had an unused license for Win2003 Server Enterprise Edition sitting around.
Using an SLK800A on it with a Panaflo L1A and it's humming along very nicely. Granted I have a 1700+ JIUHB 0310, which is one of the best non-Bartons ever, but I can do 2.0GHz @ 1.475v - doesn't break 48C folding all day. But that's a tangent...
You want small, though. If you don't like the SFF idea, then what exactly did you have in mind?