it is all about money.
windows foundation is much more cheaper than windows server with 15+ CALS.
of course, it also introduce some limitation as well.
like this server only support 1CPU, max of 8G of ram and some other limitation.
http://www.petri.co.il/introduction-to-windows-server-2008-foundation.htmalso whether QB supporting 64bit that another issue.
http://qbblog.ccrsoftware.info/2010/01/quickbooks-2010-and-2009-pdf-printer-problems-with-64-bit-systems/I personally think, if you only have 1 server in the envoironment, and only have max of 15 users in next few years, this is a good start (and the lowest investment)