Best PC for use with Flex 6700

"The difference between the 5000 series and the 6000 series is that most of the processing in the 6000 series is done in the radio, rather than the computer, thus the computer requirements are not so demanding for the 6000 series."

That isn't entirely correct.  We do not do any DSP or other signal processing in SmartSDR - it is all done in the radio.  We are however streaming audio, both DAX and with the release of SmartSDR v1.4, remote audio from the radio through the PC.  We do not do any DSP processing of that streamed audio in the client software, but we do have to shuffle it through the Windows sound system, so real time bitstream processing happening, therefore DPCs are a concern.

The generalization that the PC requirements are less demanding that for a 5000 is also not entirely correct either.  The subsystems that are utilized most are different; SmartSDR is graphics, threading and RAM centric, whereas PowerSDR is more IO throughput centric.  We are finding that the basic hardware requirements from a CPU standpoint are about the same.  

However, you are absolutely correct in your statement that having a decent graphics card is more important on a SmartSDR system than PowerSDR.  SmartSDR uses the hardware acceleration capabilities of the GPU whereas PowerSDR does not.

And the advice to try out what you have first before buying a new PC is very sound advice.  A "honking fast" i7 machine isn't necessary, but having the additional headroom it affords is nice to have.