if you're looking for a great sub-$100 PSU, i just got the Vantec Stealth VAN-520A PSU from newegg. It's $92 for a 520W 24-pin PSU with everything and the kitchen sink. It looks sharp, performs well, is quiet, has great cooling potential with the 3 fan design, and all sorts of connectors. I highly recommend it, but i've also heard great things about the other PSUs mentioned, so it's an issue of what brand you prefer and what your price range is. I personally think that the newegg price on the vantec is an absolute steal, considering it's $50 cheaper than most of the other ones mentioned, and is also about $60 below the vantec price from other vendors.
Whatever you do, you should probably get a 24-pin PSU. Have you noticed that PCIe boards from Intel and AMD are all starting to use 24-pin connectors? It's to add stability to the boards with the increased power draw of PCIe (much higher than an AGP/PCI combo). If you're gonna be investing in a 500+ Watt PSU, make sure it's well equipped for the future. Also, most SLI boards say that the only way you can use a 20-pin PSU is if you use something akin to EZ-Plug to add stability, and even then Asus recommends against anything higher than a 6600GT SLI setup. If you're looking into a dual 6800 setup, you NEED something better than a 20-pin PSU. Oh, and as for PCIe connectors, 6600GTs don't need them, but 6800 series cards do, so you'll need some adaptors for most PSUs. I don't know about the other PSUs mentioned, but i know the vantec has tons and tons of molex connectors, i had plenty left over after all my case fans and EZ-Plugs and what not, so if you spend a few bucks on some PCIe power adaptors, you should have adequate molex plugs to put them into.