There's no life changing events with USB 3.0. I have 2 external drives that use it. I don't even notice as most of the transfers are not that big, like say 2 Gigs. So USB 2 is just fine. I would not bother looking for the PCI cards. Save the $20. 
 
Another thing is so many devices don't even work with the USB 3.0 ports. I couldn't figure out why my Yamaha DTX drum driver wouldn't install until I swapped it to a USB 2 port and it worked. 
Now I'm wishing my MOBO had more USB 2.0 ports! It only has 4 so I'm having to use a Hub already. At this point only external Hard drives seem to work with the USB 3.0. 

I put in a no name PCIe X1 usb 3.0 with no problems. No molex connection on it. That's what sounds scary to me. I have a 2TB external 3.0 with its own power source, as they all are, so no need for buss power. I do have 3.0 on the mobo too so no problem there. Everything I have works on both pcie and mobo usb 3.0. I removed all pci cards and that's when I stopped getting any wierdness. No pci usb cards with devices in use while Sonar is open has added to my stability a lot. Should it be that way? I just know for me, it is.

2:43am- Check out Jim at studiocat.com if your thinking about getting parts to upgrade. He has an advise service that would maybe save you a lot of headaches with compatibility problems. I've had advise from him on every DAW I ever had or built. Stand up guy.

This past year I worked on a number of new computers with usb 3 ports.
 
Creating disk images was way quicker with a 3.0 external disk than with the 2.0 disks.
 
I purchased a couple of 3.0 enclosures, moved the disks. Now it is feasible to visit these client's offices and create new images in about 20 minutes.
 
I have had to use images to replace dying disks for 2 clients in the past 3 months. Very happy to have had them.  The more frequent I can create such the more comfortable I am.
 
I am not ready to replace the current music computer. And since I create an image about monthly, before any major windows updates and any new programs, it will be gooder to be able to create an image faster.
 

Yes on the 3.0 transfer of LARGE files. Small ones not so much difference. But when your transferring a restore image file, that's in the 100's of gb, USB 2 is SLOWWWWWW

Rather than messing with a USB card which may or may not work on your machine and then adding on the extra cost of a USB 3.0 mode external drive why not just fit a hot-swap bay to your desktop?
 
Providing your Mb (BIOS) supports hot swapping it will be as fast as your internal drives anyway and you can stick whatever drive(s) you prefer in it rather than being stuck with what comes packaged in an external box.  Actually even if your machine doesn't support 'hot-swapping' you can always do your 'swaps' when the machine is powered down.  For backup purposes that's my preferred method of working with a bay anyway.
 
Cheap, fast and effective.  It works for me.
 
No good if you want a portable bus powered drive for your laptop of course but for backing up a studio desktop it works like a champ.

Well it's pretty cheap to add a couple of 1 or 2 TB Sata 7200 drives to a desktop. Just make sure you have a good Power supply.