Well, this really depends on your budget.
I would recommend against USB for this at all, because it's not as stable, in general.
The cheapest solution is to get an ASMedia or Marvell based controller card, with eSATA ports (the ASMedia one requires the specific drivers for it to be installed, not just the generic Windows drivers).
As for external enclosure, this depends on the card in use, and your preferences. Though, I'd recommend something like the ORICO 3559SUSJ3-SV enclosure (5 bay, aluminum case, eSATA). But you could use something like the MediaSonic ProBoxes (though, we recommend purchasing spare power supplies, as they seem to burn out rapidly)
The other choice is to spend more money and go more of the enterprise quality.
I suspect that isn't the case (especially as this gets expensive QUICKLY).
But a LSI card or a HighPoint card, with a "breakout cable" for 4 eSATA ports (each supporting multiple eSATA enclosures), or use a "SAS Expander case" to add many drives.