If you are looking for a mini itx system there are several different types. By far the majority of the mini itx systems use the VIA built in processors. For the most part these top out now at 1200Mhz and run about $250 for the mobo and PCU only. You can find some variation in the CPU as there are some mobo's that support celerons and some that support PIII's.
mini-itx systems are usually pretty expensive. I have one system that I am likely going to be putting on eBay tonight since there was no interest in it on this board.
Here's the one I am selling ---->
mini itx.
The mobo in it is a 800Mhz and new they run ya about $180.
Flex and micro ATXsystems are more cost effective when you weigh in that you can use more CPU flavors, but you wont find a flex or micro ATX board that will be 6" x 6" in size. So if size is a consideration; mini itx is for you
If you are loooking for a project such as a PVR, 800Mhz is plenty provided you have a hardware card that handles encoding like a Hauppauge 150, etc.
If it is MAME, then of course your limit is only to what games you want to play. Most NEOGEO games including MSLUG5 play well on a 800Mhz, but you'll get more playing games on a 1200Mhz or a P4 / AMD solution.
So it really depends on the project.
-Goz