I am thinking of getting a PCI-E usb3.0 card for my computer. Do I need special USB3 sticks and cables to harness the speed of USB3? Will my USB(2?) stick work at USB3 speed?
Just to clear up some possible misconceptions about USB.
First there are four speeds. There is low speed of 1.5 Mb/s (USB 1.0), full speed of 12 Mb/s (USB 1.0), Hi-Speed of 480 Mb/s (USB 2.0) and SuperSpeed of 5.0 Gb/s (USB 3.0).
People tend to confuse speeds and versions of USB standard. Usually devices are made to the newest standard even if they don't need the maximum speeds provided by that standard. There are lots of USB 2.0 devices which use 12 Mb/s or 1.5 Mb/s because they don't need higher speeds. As USB 3.0 spreads, there are going to be lots of USB 3.0 devices which won't be using the SuperSpeed introduced with USB 3.0 standard. So while OP probably considered 5 Gb/s as speed of USB 3.0, all other speeds are also USB 3.0 speeds.
Another point are the cables. USB 3.0 provides new types of connectors and cables which are going to be needed for SuperSpeed use. USB 3.0 cables are mostly compatible with USB 2.0 connectors, but there are couple of differences. USB 3.0 B male connector is not compatible with USB 2.0 female devices and can't be plugged in!
The third point are unit loads. Standard unit load for USB standard up to 2.0 is 100 mA and single port can provide up to 5 unit loads. For USB 3.0 it has been increased to 150 mA and up to 6 unit loads per port. That means that USB 3.0 devices are going to be expecting more power from computer. On the plus side, that means that USB 3.0 compatible phones and other similar devices will be able to charge faster, but on the other hand, poorly designed USB 3.0 devices may have too little power to work if they are connected to USB 2.0 port. This most likely isn't going to be a problem in the transition period, but later it could be.
So in the end, if you want to have for example a flash drive which uses USB 3.0 SuperSpeed, you'll need USB 3.0 port on the computer (and if it is on an expansion card, the card will need to use PCI-E), USB 3.0 cable which will connect drive and port and USB 3.0 drive which supports SuperSpeed. If one of the 3 components isn't USB 3.0 rated, it will slow down the whole system.
In Theory when using a USB 2.0 device (Memory Drive) it should operate at the maximum speed the device can give, in most cases this will result in some performance improvement, due to the fact that USBIF states that USB 2.0 devices in MAC's consistently only give 30% of the raw data transfer, and in the Windows Machines, 50% of the raw data rate. this is due to the Process Handling Protocols used, and is considered "Acceptable" by the global governing body who set the USB standard..
Personally it is not acceptable to loose 50% of the performance capability of anything. if I studied at school and only got 30%-50% percent of my exams completed and 30-50% in my resulting grades, would I be an Acceptable Doctor? or Aeronautical Engineer? I do not think so