Need some advice on Microsoft Exchange migration

Background:  currently running Windows SBS 2003.  We have a single domain and the SBS runs our ISA and Exchange.  Last fall I moved our SQL off the SBS to its own Windows 2003 Server box.  I have approx. 130 users who share approx. 25 workstations.  My current project is to move Exchange off the SBS to its own box.  We are a not for profit company and I do have access to Windows 2003 Server Enterprise Edition very cheap but I'm going to have to go out and buy Exchange.  I would appreciate some advice on what my options are regarding this.

1) My hardware is not 64 bit and it is my understanding that is required to run Exchange 2007.  If so, then will Exchange 2003 standard edition be sufficient or will I need the enterprise edition?  

2)  How will the licensing work for this new exchange server?  I'm a little fuzzy on how I have to license considering everyone in the organization uses Outlook Web Access.  It seems to me licensing the devices will not work because of the Outlook Web Access?   Do I need to have 130 uesr Cal's then?

After I move Exchange off the SBS to its own server then my next project after that will be getting rid of the SBS.  Our budget is small and I have to take things in small steps.  Any advice on this issue would be greatly appreciated.