Hi,
Here's the system I am planning to upgrade to but I need to know I will be able to OC the 2500+ to 3200+ w/400fsb:
 
Shuttle XPC Barebone System for Socket A at 333/400MHz FSB AMD CPU, Model SN45G (NVIDIA nForce2 Ultra 400 + MCP-T)
 
Barton 2500+
 
Corsair Value Select 512mb ddr pc3200
 
My main concern is being able to overclock in a SFF case, since there is only one 80mm fan that handles cooling duties.  Also, would the value select ram be up to the task, or would I have to get XMS?

i would go for the 2500+ M (mobile) cpu.  It runs a lower core voltage by default and wont require as much voltage to carry out a mild to moderate overclock.
 
If it isn't a P6 then it isn't a processor
110% BX fanboy