It's a shame you picked a Dell, with their annoying use of proprietary connectors for the USB it makes expansion a lot harder than it should be, my Lenovo m57e SFF (presently used as a media server and IP Cam NVR) has two USB2.0 headers on the motherboard, one occupied by the front USB sockets, but the other header is spare, and they're standard pin headers, so could probably use something like this inside the case:
Code:
http://www.ebay.co.uk/itm/261529641115
There are others which are a PCB with two USB sockets on, great for use with USB sticks for USB-booting something, but probably not a good idea to use for something on the end of a wire as it could get snagged and damage the motherboard...