How add USB 3.0 to Optiplex 755 Desktop with Win XP Pro?

I have a good-condition Optiplex 755 DESKTOP with Win XP Pro and would like to add USB 3.0 to it.  How is that done?

Right now, I already have two cards in place at the bottom back - a video card and a low-profile fax modem card.   Since I have the DESKTOP model Optiplex (NOT mini-tower), I probably need another low-profile card for this, but not sure.

In the front of the machine, there's space between the hard drive and the DVD because I do NOT have a floppy disk drive,

Please point me in the right direction as to what to buy. 

FYI - in a different thread, rohe advised as follows:

Without the card riser, you can only use low profile cards in either the PCI or PCI-e slots. If your video card is in the PCI-e x16 slot, you either have to install the card riser to add another PCI-e slot or use a PCI card in an empty PCI slot.  Check the manual for details about the versions of PCI and PCI-e used by this system and make sure any card you buy is backward compatible to support those versions of PCI and PCI-e.

Rohe's suggestion is interesting, but I have never actually seen the card riser, and the drawings in the manual are totally unclear as to how it works.  Extremely poor graphics by Dell.  So if the solution requires the riser, please also point me to some pictures that show visually how it works.

Thanks.