Server 2008 Versioning
Ben is entirely correct on the main reasons to purchase Enterprise over Standard. I would still recommend Enterprise (or even Datacenter, depending on your usage), however. The reason is virtualization.
MadBoy is correct about the licensing. You can either install it on the bare metal and have up to four hosted copies of the OS inside something like VMware Server or Virtual Server, or install a hypervisor such as Hyper-V or ESX and put your four installs (not five!) in there.
With Datacenter edition, however, you can have unlimited virtualized instances of Server 2008 (any version, up to and including Datacenter) for all the licensed CPU's you've purchased (keep in mind Datacenter edition is purchased in two-CPU SKU's). If you're using fairly recent, high-memory physical server hosts, you can easily fit 10-20 or more moderately used servers per dual quad-core 48GB host. You may not think you need this much, but you can quickly get to that number when you think of having two domain controllers for the forest, two more for the domain, an additional for a lag site, several PKI servers (makes Exchange 2010 easier to work with for certificates), one or more file servers, etc.
It's well worth taking some time to think how big this IT infrastructure is going to get. Rushing forward with Standard edition can hurt you in the future in total cost.