I have a Mac Pro 5,1 running 10.7.5.
Do any of the USB ports support USB 3? Or do I need to buy a card?
No the first Mac Pro with a USB 3 port is the latest (6.1, late 2013)
From the specifications for your particular Mac Pro:
Connections and audio
- Four FireWire 800 ports (two on front panel, two on back panel)
- Five USB 2.0 ports (two on front panel, three on back panel)
- Two USB 2.0 ports on included keyboard
- Front-panel headphone minijack and internal speaker
- Optical digital audio input and output TOSLINK ports
- Analog stereo line-level input and output minijacks
- Multichannel audio through Mini DisplayPort
A quick Google search shows several PCI Express USB 3.0 cards, e.g.,
I know this is a bit late, but the Inateck cards using the FL1100 chipset from FRESCO work great because that's the same chipset used in later Mac Pros. Here is a guide to installing these cards