Question : Which version of Server 2008 should I purchase with my new server?

I am on Dell's website trying to configure a new server for my office.

We tried SBS 2008 for the last two weeks and decided it was too limiting for our organization of 20 users.

On Dell's website, I have a choice of:

(a) Windows Server 2008 R2, Standard Edition,x64, Includes 5 CALS
(b) Windows Server®2008SP2, Standard Edition,x86, Includes 5 CALs
(c) Windows Server®2008SP2, Standard x64, Incl Hyper-V™, Incl 5 CALs

Our goals for the server are:  
(1) Share documents that reside on the server
(2) Run Exchange 2007 (or Exchange 2010)
(3) Ability for each user to access there own office desktop computer remotely
(4) Access to e-mail and server documents remotely

Which Server OS do you recommend?

Answer : Which version of Server 2008 should I purchase with my new server?

I would say that option A is your best option if you have quite a bit of RAM in the server (>4GB), you don't need hyper-v unless you intend on using it.