I recently purchased the HighPoint RocketU 1144AM PCI-E 2.0 x4 Quad-Port USB 3.0 Raid card for my Mac Pro. Since most external drives you buy these days are USB3, and the Mac Pro only has (and perhaps only every will have, if the rumour mill is true) USB2 ports, the only option I had for expansion was a USB3 card. The Mac Pro doesn't have Thunderbolt on it, and the external drives equipped with Thunderbolt are prohibitively expensive right now, so USB3 looked to be a better bet.
The card cost me under $150CAD, including shipping from the US. This card is pretty sweet; it has four ports on it so you can have four external drives, but beyond that you can also set them up as RAID0, RAID1, RAID5, or RAID10, and JBOD. There also seems to be a way you can bond the four ports together if the cables all go to the same storage device (I don't know of any such devices at the moment though, and I imagine they're probably right up there in cost with a Thunderbolt storage device). Individually, the ports are 5Gb/s, bonded you can supposedly get 20GB/s.
So I did a few tests, and this card has seriously impressed me. I copied a 2GB file around in random ways to measure the speeds:
Talk about wow. Note it was the exact same drive, first connected to a USB2 port and then connected to a USB3 port. The speed increase on USB2 to USB3 is insane. What's even more astonishing to me is that the transfer speeds were faster to the USB3 drive than to another internal drive. I suppose if the guts of this thing had SATA3 it would be a different story though. Since I don't think I'll be getting SATA3 in this thing anytime soon (and since I have run out of space for expansion cards due to the second double-high video card I've got in here), I suppose I'll have to live with it. All in all, I think this was a good investment to make. If I run out of room internally, I don't have to worry about just using external drives for "sometimes" access, but I imagine it would work well enough to run a virtual machine off of.