the fin design really determines the temp. the heat will eventually overcome the rad and it will just get overloaded and the water temp will just keep rising.
what fan are you using for the multi-system?
--Matt
p.s. I am now starting to think of water cooling our bewulf cluster at school if I can get the funds

heheh, 10 WC via c3's would be nothing for a heatercore. but it'd be a waste since they make like no heat and it'd be enpensive. -- hmm idea gunned down