I run a i7-2700k overclocked to 5.0ghz liquid, 32GB ram, 2x256GB Crucial M4 SSD's in RAID 0.
There are two issues here. Clock speed and multi-threading.
There are some bits of code NinjaTrader does a good job at multi-threading, and there are some it does a bad job. I've talked to Ray about the bad job plenty, he is fully aware, and NT8 will likely offer significant improvements in this area.
If you have a notoriously bad multi-threaded app, the answer is to have faster clock speed since the app won't take advantage of multiple cores anyway. But as the app improves, then more cores is better.
For the standard home user, a config like mine is more or less top of the line. I've built quad Opteron's, I've got a Xeon sitting idle in the other room, and both of them are slower than my i7 simply based on the ability to overclock my unit to 5.0ghz. So for all the times only a single thread is being used, my i7 is screaming along much faster.
You can build a system like mine for pretty cheap, check out newegg.com.
The CPU speed matters most. Any system with 12GB of memory or more is likely enough for 99% of people here. A single SSD is likely enough for 99% here as well. No need to go overkill and waste money unless you are an enthusiast.
That said, there are always new things coming along. I've been looking at software called IQBroker lately, and I've seen memory utilization north of 24GB before with this app as it spun through the SP500 index simultaneously with enormous data sets. So it all depends on what you are trying to accomplish.