Need to prevent upload or any thing else to secure data ?

my company have 2008 domain network – the users have to work on critical data and we need to make users can’t leak this data over internet , our users Reliable of them but for reservists , we think that we can do that by prevent them from upload, we can’t prevent them form net they need it to Handel their work … can we do that , we have domain network server 2008 , exchange 2007 and ISA 2006 . exchange is published on net and user get net access throw ISA ..
I need to ask if we can do that by any way else ? and is there technology to control rights to non office file as AD RMS do that ?