Andrew, I'll give you an answer (probably equally general to your question). If those 60 users are all sharing one server today and the user count will stay in that range (ie not dramatic growth over life of your server deployment), you are probably ok staying with one server and the new server would certainly boost performance across that network of users - can always add a 2nd one later if needed.
Having a hyper-visor technology on the new server will help you add deploy any new software and services more quickly on the new server and if you get to a more than one server environment, you can move applications from server to server with supporting virtualization software capabilities.
If you move to Microsoft Win 2008 a hypervisor technology called Hyper-V comes with the OS and supports core virtualization functionality and would likely get you started without a huge investment - then you can evaluate if you need to invest in virtualization. There are also basic versions of other hyper-visors (VMware, Citrix, ...) that can be obtained and loaded onto your new server without a huge investment.
I would certainly encourage you to look at what is out there from a hypervisor support on the new server, however, I don't think you NEED it based on the general description of your environment
my free $0.02
If you would tell us which kind of server (CPU and memory) you are using in the moment and if you have performance problems or not in the moment would be useful to give you an usefull answer.
As you maybe know, the "old" SBS 2003 server from Microsoft handled all the roles you talked about on a single system and even a series 3000 Xeon might be fast enough, especially if you help him be using an add on raid controller which gives you a better performance than the on board solutions because of dedicated cache memory and an own raid processor.
OK, here's the spec
Intel Xeon 2.66 GHz Model 2 processor 533MHz Bus 512kb L2 Cache (showing up as quad core but i think its actually 2x dual core)
Intel se7501cw2 motherboard
2 Gb DDR PC2100 RAM
SCSI Raid Card
4x 150Gb HD in Raid10 = 465 GB usable capacity
1x Intel pro 100 NIC
1x Intel pro 1000 NIC
I was looking at possibly upgrading to one of the new Dell Poweredge R410 or HP Prolient DL320 G6 Servers with the Intel 5500 series processors.
The HP Server should be fast enough for next years, but i have one tip: If you have enough place in your rack, think about using a ML server instead of a DL. Why? The bigger server are easier to keep cool and the don´t make so much nois....
Based on your description ... you have a 2x single core Xeon server (http://ark.intel.com/Product.aspx?id=27272)
Either of the two systems you mention (Dell Poweredge R410 or HP Proliant DL320 G6 Servers with the Intel 5500 series) would be a logical upgrade for your older server.
I think you'll enjoy and benefit from the boost in performance and energy efficiency on these new servers.