Hmm.. Not too significant, as the Pally cores are a fair bit hotter than the Barton anyway. I don't have any experience with these SFF boxes, so I don't know what the airflow through the case is like, but I would assume you'll probably even get a decrease in temp.
Why don't you just buy a mobile Barton 2500+, and run it at 3200 speeds? You have a good chance of getting it there on a fairly low voltage, which = less heat. Plus of course it saves money.
Do you have PC3200 RAM though? You'll need it for a 3200+ (or a 2500+ overclocked to 3200+ speeds) Plus of course your motherboard needs to be able to handle the 200Mhz (400DDR) Bus speed too.
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Epox 8RDA+ rev1.1 w/ Custom NB HS
XP1700+ @205x11 (~2.26Ghz), 1.575Vcore
2x256Mb Corsair PC3200LL 2-2-2-4
Sapphire 9800Pro 420/744