I hear good things about the OCZ units being relatively silent, but can't speak from experience there since I don't have one. I can tell you they're very efficient with huge heatsinks, which is one way to make them quieter - less heat means the fans spin slower than less efficient units at the same power level.
Here's a review of the SilenX... this isn't a terribly good review, but will give you an idea what the PSU looks like and what connectors come with it:
http://www.planetamd64.com/index.php?showtopic=4470
The main differences between the OCZ 600w and the SilenX are:
-Single 36A 12v rail on the SilenX, compared to dual 12v on the OCZ
-SilenX is Fortron built, OCZ is Topower built
-SilenX unit is lacking PCI-E connectors, but will work with adaptors
-Single fan on the SilenX vs. dual fan on the OCZ... claim is for 14dB, but I don't believe it for a second. Might be close, but 14dB is stretching it.
-SilenX lacks externally adjustable pots, unknown if Fortron built any into the inside (but they probably did)
-No power switch on the SilenX
-Active PFC on the SilenX, none on the OCZ (I think - someone correct me if I'm wrong)
-OCZ probably has a better warranty
-The SilenX has no 12v advantage over the more readily availible Fortron FSP550-60PLG.
Some commonalities... both units are massive overkill for any standard PC configuration I can think of not involving pelts. Both claim over 75% efficiency, but not sure if either live up to it. Both should be fairly heavy. Both should laugh at SLI current needs
The only other difference I can think of to point out is that being FSP made, the SilenX will likely have a long term reliability edge. I say likely, because if that silent fan turns out to be insufficient at high load levels, the SilenX's life will be shortened. We'll just have to trust SilenX on that one, I guess.