Using a squeezebox will not allow you to use use the Foobar interface. The squeezebox uses a proprietary interface now knows as "squeezecenter", which is actually quite nice. You can test out the interface for yourself before buying a squeezebox: download "softsqueeze" from the logitech site, and it runs the interface as a straightforward music player for your PC.
As for finding the best transport, it's anybody's guess what will work best.
The squeezebox is nice (I used one for a few years). It caught peoples' imaginations because it was relatively inexpensive to upgrade, either with improved power supplies, or internal modifications. But poor power supplies, wireless interference, and even interference from ints own internal features (like the display) can pollute your sound.
Running your PC through a USB-to-SPDIF converter like the USB thingy is an option. But beware of a few things. Many of these inexpensive converters are inexpensive for a reason. You're probably better having one that uses its own power supply, rather than running off power supplied through the USB connection, as power supplied through the PC is notoriously noisy. Many people have had success using the converter from Trends, which has a price very similar to the USB Thingy.
Be aware that this approach gives you countless sources of jitter and noise. Your PC power supply and internal components potentially degrade your sound. The way the operating system handles music may degrade your sound. The particular USB architecture can degrade your sound. The audio drivers you use can degrade your sound. The power supplied to the converter may degrade your sound. The converter-chip might be crappy. The RCA connectors may not be true 75ohm connectors, and the spdif protocol has all sorts of problems.
Unfortunately, heading in a different pc-based direction usually means just trading one problem for another.
Currently, I have a cheap netbook with solid state drives running foobar. It's got a wired ethernet connection to remote network storage where the flac files are kept. I use a USB dac. I think it sounds great, but I'm sure it could be improved in many ways.
Even sticking to the two tools you have now, there are many experiments you can do to see how things affect your sound. Try the pc based solution through a different usb port; try it through a laptop or desktop instead; try it with the battery powering the laptop; mae sure you're using an ASIO driver, or have implemented kernel streaming. The list goes on.
Good luck.
Chad
