Question : server 2008 capacity

hi ,i have server 2008 standard sp2 with intel Xeon E5405 2GHz processor and 12GB of RAM
this server function as mail server with exchange 2007 ,DC,and file server,
there is a 100 users that connect to this server

my question is if i can install the "blackberry enterprise server express" without any hard performance issues ?

Answer : server 2008 capacity

I would not install BES on that server. You are already going againist Microsoft Best Practices by running exchange on a dc. I would definetly run it on a secondary machine.