Yes, I'm currently steering towards a FlexRadio running custom DttSP (or perhaps GNU Radio) software. PowerSDR looks like it's basically a nice GUI on top of DttSP.
Also, I just thought I'd relate my stream-of-consciousness findings after digging into the FlexRadio a bit more (probably most of this is old-hat to folks here):
* Out of that you get two base-band signals (I and Q or I/Q, for in-phase and quadrature or orthogonal, basically a complex number).
* The FlexRadio samples those signals at fairly low base-band frequencies.
* So you can do this kind of thing with a sound card (e.g., the Softrock) but it's fairly low quality -- the FlexRadio receiver is basically higher fidelity and uses higher bandwidth ADCs (192KHz), but still not MHz-frequency ADCs like the USRP (32MHz bandwidth).
* As the page at
http://www.flex-radio.com/Products.aspx?topic=faq says, "Once the I and Q signals are digitized, DSP algorithms perform all of the demodulation and signal enhancement eliminating the analog (and sometime digital) circuitry found in traditional radios which provide the same functions. Transmission is just the reverse of the RX process. There are no multiple IFs in a SDR."
* This document, written by the FlexRadio folks back when they were starting, has a fairly good explanation of the maths behind QSD in the section "From RF to a PC's sound card":
http://www.flex-radio.com/Data/Doc/qex1.pdf
So the FlexRadio is ready-to-go for HF radio transmit and receive, but basically is only good for HF a bit above (up to 60MHz carrier freqs). The USRP is much more general purpose and uses much higher bandwidth ADCs (and has an FPGA), but isn't nearly so ready-to-go for HF stuff.
Cheers,
Ben.