I bought a PCIe USB 3 card and put it into a 5 year old HP desktop, and it works fine. All you need is a PCIe X1 slot, but you can use the wider PCIe slots if desired or needed. My card's an Inland I think I paid around $30 for. It works just fine with Vista, Windows 8.1, and the pre-release Windows 10.
Good luck.
So I have a Dell Studio XPS 8100, I believe. Late 2010, great PC for its age (graphics card isn't the best, but otherwise good.)
-Core i5 650, 3.2GHz
-ATI Radeon HD 5450
-8GB RAM
-WD 1TB hard drive (partitioned with Windows 7 and 8, although I really never have used the 7 partition.)
(Research shows that this is a Sata-6, 7200rpm HDD)
I will be off to college in a year and a half, so I'll get a new laptop then. In the meantime, I want to bring this PC up to date a little. It's been a great machine, but I've been gambling enough with essentially no backup, so I want to begin the process of backing things up.
So, to help me with my new external hard drive (a replacement for one that crashed, I think I'll use online backup too as a "backup to the backup"), I want to get a USB 3.0 card. The issue is that my PC apparently has few upgrade slots.
There are 4 slots on the back, and 2 are taken (from top to bottom, it is taken-free-taken-free I think)... I think the PCI-e x16 slot has the Radeon in it. Another slot has a Wi-Fi card (which I do not use anymore, rather using a long Ethernet cable.)
I am very unfamiliar with expansion slots, and I want to know whether my PC has the power supply cables and free space to support a USB 3,.0 card. The PC has never been opened before (at least not for upgrades, IDK about cleaning.) Can anybody with some more knowledge of this system enlighten me?

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