You want to add USB 3.0 to your existing computer. There are lots of add-on cards for USB 3.0. What should you buy?
USB 3.0 runs at 4.8 Gbps (Gigabits per second). A gigabit per second usually works out at 100 megaqbytes per second, allowing for some overheads. 4.8 Gbps is about 480 MBps (MegaBytes per second) or about 10 times faster than continuous data transfer from a magnetic disk.
Why do you need all that speed? Modern disks have an on board memory cache that can transfer data up to 300 MBps. When you copy data from one disk to another, you can have bursts of data transferring at 300 MBps and you need that speed. If you backup files to an external disk while reading files from an optical disk and both are on a USB 3.0 hub at the end of one USB 3.0 cable, you could be using bursts of data transfer up to 400 MBps.
SSD (Solid State Disks) can transfer data up to 250 MBps with some expensive disks exceeding 300 MBps. Think about a computer with a couple of external SSD devices. One contains all the data for your one big project that makes you a lot of money. The other disk contains continuous incremental backups of the project. You want both connected through USB 3.0. You need an expansion card with 2 USB 3.0 sockets so you can have one socket dedicated to each disk. The total data transferring through the expansion card could be 500 or 600 MBps.
Expansion cards plug into PCIE slots. PCIE is also written as PCIe. PCIE comes as version 1.0 and version 2.0 with version 2.0 twice as fast as version 1.0. There are also the variations x1, x4, x8, and x16 with x4 four times as fast as x1, x8 eight times as fast, and x16 sixteen times as fast. There is usually only one x16 slot reserved for your graphics card.
PCIE 1.0 x1
A PCIEx1 slot transfers data at 250 MBps which is fast enough for two magnetic disks continuosly transferring data but not enough for two disks bursting out data from their cache or from two fast SSD devices. PCIEx1 cannot run either USB 3.0 socket at full speed.
PCIE 2.0 x1
A PCIE version 2.0 x1 slot transfers data at 500 MBps which is fast enough for two magnetic disks continuosly transferring data or one SSD but not enough for two fast SSD devices. PCIE 2.0 x1 can run one USB 3.0 socket at full speed but not both.
PCIE x4 is the next step up from PCIE x1 and transfers data at one GBps (1000 MBps). One GBps is slightly faster than the maximum speed of two USB 3.0 ports. You want your USB 3.0 expansion card to work at PCIEx4 to get the maximum USB 3.0 on either USB 3.0 connection.
PCIE 2.0 x4
PCIE version 2.0 x4 transfers data at 2 GBps (2000 MBps) which is fast enough to run four USB 3.0 ports at full speed.
Some expansion cards
Gigabyte GA-USB 3.0
The Gigabyte GA-USB 3.0 has two external USB 3.0 ports on a PCIe x1 card using an NEC USB 3.0 controller chip and a 4-pin power socket for additional power. The 4 pin power socket is the most convenient way to add the extra power required by USB 3.0. The PCIe x1 connection will not let either port reach maximum speed.
The card sells for AU$38.90 at one online shop.
Shintaro USB 3.0 expansion card
The Shintaro USB 3.0 card sells for AU$38.25 and is only PCIE x1 but is PCIE version 2.0 x1 giving you enough speed to run one of the USB 3.0 ports at maximum speed. The extra power for the USB 3.0 ports is delivered from a floppy disk power cable connection, something you might not have on a modern power supply.
Vantec UGT-PC302 2-Port SuperSpeed USB 3.0 PCIe
The Vantec UGT-PC302 2-Port SuperSpeed USB 3.0 PCIe card says it is PCIE but does not specify which PCIE. The extra power is drawn from a SATA power connector, something you might not have on older power supplies. At AU$58.30, the card offers no advantages.
Welland Turbo Leopard SuperSpeed USB 3.0 & PCI Express 2.0 card
The Welland Turbo Leopard SuperSpeed USB 3.0 & PCI Express 2.0 card uses PCIE version 2.0 making it equivalent to the Shintaro and better than the other choices. The current price of AU$46.90 might encourage you to look at the Shintaro first.
Asus U3S6 PCIEX4, SATA3 6G, USB3.0
The Asus U3S6 PCIEX4, SATA3 6G, USB3.0 card is listed as an expansion card for Asus motherboards. I do not know if it works anywhere else. The OEM version, with no cables or fancy box, is only AU$34 and worth a look if you like to experiment. You get PCIE x4 which is fast enough to run both USB 2.0 ports at full speed. You also get two internal SATA III three ports which could be handy. The card is big, full height, and might not fit some small space saving cases.
I looked at this card for a NAS device because I needed two extra SATA internal connectors for a motherboard that had only one spare expansion slot and having the extra two USB 3.0 connections would be handy for additional backup. The space saving case requires low profile cards. The Asus card would not fit the case.