I hope you mean BES Express 5, BES 5 is not supported on SBS.
Your CPU specs are a bit on the low side. Do you have at least 1.5 gig free ram?
http://na.blackberry.com/eng/service...quirements.pdf
You can not upgrade to BES express.
http://supportforums.blackberry.com/...-0/td-p/454694