Not quite. Everything bears out except that "320 bpm = 320 notes per second" bit. 320 beats per minute means just that - he's squeezing 320 quarter notes into every minute.
Aside from that, you're right - 320 beats in a minute, times four notes a beat, does give you 1,280 notes a minute. Divided by 60 seconds, and you come out at 21 and a third notes a second, pretty damned fast.
I think it's easier to understand the math from the other perspective - at 60bpm, one beat a second, you're playing four notes a second. So, figure out the ratio between the tempo you're playing at and 60, and multiply that by 4 - in this case, (320/60)*4 = 21.33 repeating.
MSHILARIOUS - My picking is pretty crap (I'm more at home playing legato) and it's especially bad playing scales across the entire six (or 7 in my case) strings of the guitar and back, as the string switching is what screws with me. I'm fine for a string or two, but when I start having to shift my hand to get to the next string, my accuracy suffers... That said, I've been working on this lately, and last night I had the metronome up in 140-150bpm territory and was running through a couple stock "box" position diatonic scales relatively cleanly... On the conservative end, that puts me at just shy of ten notes a second. That said, if I'm confining myself to fewer than 2 or three strings at a time, it's been a while since I've just gone balls out for top speed or anything but I should still be able to get over 180bpm, for 12 or so. And, if I throw picking to the wind and stard doing things like 4 notes of legato with a tapped note up top in a fixed position on a single string (i.e - sacrificing everything musical for raw speed), I'm not really following any particular rhythmic pattern by this point so the metronome angle goes right out the window, but someone started a cock-measuring contest over at Tabcrawler about this, and based on averaging the number of distinct measurable attacks per second over a couple seconds, I seemed to come out in the low 20's. Still, that's not music (not because it's too fast, but because I'll freely admit that my technique is limited such that I couldn't do anything musically interesting at that speed), and it shouldn't really count - about 10-12 notes a second is, at this point, about the limit I can play anything musically interesting.
(and now I'll wait for this thread to TOTALLY degrate into a pissing match, lol)