I only have a SOHO situation, but Snort runs just fine for me on an Atom CPU.
I put 8GB of RAM in my pfSense box, because I could. ; ) But, it typically uses only a very small percentage of that, typically 8%.
Bottom line, even when running Snort, pfSense has a very low resource requirement. Obviously if you have an enterprise environment, or if you have a gigabit fiber connection, you'd want a CPU a little beefier than an Atom, but an i7 is completely over the top.