The sweet spot for PSU's is the 650 watters as they are often comparably proced to 50 and 600 watters. Since PSU's hit peak efficiency at 50% load, the extra headroom provides both a cushion and better eficiency.
For comparison purposes , the list below contains performance ratings (10 scale) at jonnyguru.com / prices as per newegg on 02/26/2010 and ones w/ double asterisk (**) made it onto silentpcreview.com's Editor's Choice List:
Note: Just because you see a brand name on this list, never make the assumption that their other lines are of matching quality. Silverstone for example has other models w/ 6.0 and 7.0 ratings.
Antec Signature 650W (10.0) OOS **
Seasonic X-650 650W (10.0) $160 **
Silverstone Olympia OP650 (10.0) NFS
Thermaltake Toughpower QFan 650W (9.5) NLA
Antec Earthwatts EA650 650W (9.0) $75
Antec True Power Trio 650W (7.5) NLA
XFX XPS 650W XXX Edition Power Supply (7.0) NFS
Antec NeoPower Blue 650W (6.5) NLA
BFG BFGR650PSU 650W (6.0) NLA
NorthQ Black Magic Flex 650W PSU (5.0) NFS
NFS - Not For Sale / NLA = No Longer Available / OOS - Out of Stock
If I was to get an OCZ, I would go for the Z series