Trying to upgrade my gaming computer and need a little help.

[Alphalegend]

So my computer is built from scratch on a low budget, but now I've come into some cash and want to upgrade, but being as it was my first real computer build I'm still a little lost on what needs upgraded and what does not.

Specs:

Intel i5 2500k 3.3GHz
Gigabyte
GTX660 Ti video card
XFX 550W power supply

It has 8gb Ram and 500gb memory

My biggest complaint is the lack of usb support for my motherboard. I have 2 usb in the back and only 1 usb in the front. Not very good at all. I'm looking for more like 6 usbs. Another thing is I've been thinking about trying to SLI instead of purchase a better card. Thoughts?
Suggestions for a motherboard?
Anything? lol

Thanks for the help.

Edit: I forgot to say that yes my motherboard supports 6 usb but the way they have them set up my mother board is crowded by the gtx 660 ti and I can't plug 3 of them in.