1) i've actually not used the 603 or seen it, however after reading several reviews i just got the idea it was simply roomier, and had a different front style (which i thought looked better, this played in my decision for the 320 as well). i figured if the 603 was so great, and they didn't change anything but a few design specs to the 320 it could only be better, not worse. more room, better style? i'll go for that
2) yes, the fong kai approved PSU is awesome, just might want to consider changing the PSU fan (which i highly reccoomend).
3) i have a modest fan on my alpha pal6035 heatsink which doesn't create much/any noise at all (the quiet adda fan from pham computer) along with a 92mm panaflo L1A running at half volt, and does the job. i'd strongly reccommend a running heatsink fan, if not a strong one then use the ADDA fan which won't create noise at all. keep in mind the fan duct isn't going to directly cool your CPU, simply the general area AROUND the cpu. the hot air above the heatsink gets pulled out through the duct, the duct doesn't do anything for the cpu directly.