I don't think that even quite a few more users will make much difference. If you get to high numbers of users you will probably run up against a band width problem before you have a problem with an i7 processor. Especially if your solutions have video or large numbers of images. You can play with the amount of memory the server uses in the setup options. More memory allocated can mean more speed but it can also mean more lost data if you crash.
Here are all the recommended specs based on usage for WebDirect including the number of cores and memory. If dealing with a two-machine deployment and using WebDirect, the bigger demand is on the Web Server over the Database Server. In a single machine, these specs would of course apply to just your one box.
According to this you should be fine with 10 concurrent users and 4 cores. At 17 concurrent users you would want to bump up to 8 cores and 12GB of RAM.
Keep in mind these are FM's recommendations and everyone's solution will vary a bit based on its unique characteristics. I lean towards exceeding these guidelines rather then fitting right in line with, especially in areas where the cost is not prohibitive.
In doing some recent research on this as I plan our next set of hardware upgrades, it was interesting to learn that 2009 XServes still beat out current gen Mac Minis. So there is still some life to be had there for that investment in some cases.
FM General Hardware Considerations for FM Server: