Is there a discernable difference in SQ between the two?
On several comparisons I've done .....I can't tell the difference....at first listen.
But with the right gear......I would say the overall presentation is the same between 320 kb/s and FLAC but with a few details...like say the texture and weight of a cymbal or the finger sounds on a guitar string....or the multi-textural sounds of a drum head come through with a bit more clarity or resolution with a FLAC.
But the short answer is 98% of the people out there on 98% of the gear thats used will not be able to distinguish between the 2 codecs.
I can only tell a difference if I use my SR-009 with the the dCS Scarlatti as my source that is running through my Weiss DAC 202 with everything connected via Nordost Supreme Reference cables. When I use my SRS-4170 or Ultrasone Edition 10 headphones and the mbl system of the 1621 cd transport and the 1611 HR DAC with the Transparent Musiclink Super cables, the music just doesn't seem to have as much bite in 320 as it does in FLAC. There is certainly just a touch of gray in the blackness. Although the mbl is a fine piece of equipment, it just doesn't quite punch out of it's weight class.
Oh wait, there is probably a placebo difference but I doubt anyone in the homo sapien species can hear a difference. Nevermind.
I think that greatly depends on the original source that you encode from. If the source is detailed enough, then yes, there is a discernible difference, and sometimes, I think I can hear the difference plain as day with good $30 headphones w/ some EQ, so equipments don't have to be too crazy, either.
Listen for yourself. I know you already have Foobar, so check out this component: http://www.foobar2000.org/components/view/foo_abx
Take one of your FLAC files and convert it to 320k. Then highlight both files, right-click -> utilities -> ABX two tracks.
You need to do it at least ten times I'd say as to rule out luck.
I think there's a graph I can plot showing the correlation between post count and the propensity to say yes
Anyways, I don't think I'd be able to tell, but I've never tried ABXing at 320kbps. I've only gone up to around 200-250 (vbr), and I could tell an incredibly slight difference but didn't really care. This was all with modern Lame versions. Ogg Vorbis and AAC/M4A encoders both perform a bit better than Lame. In full disclosure, I didn't have quite as good of equipment as I do now, back when I did said test.
I agree with everyone else that there is a difference but its a slight one. At least for me I just mainly use 320 because I can't fit more albums and mixes on my pmp. Most of my albums are around 150mb give or take 10 to 15mb, were flac is more like 450mb and theres not enough of a difference in SQ for it to take up that much more space.