Molex connector on Mobo

Okay I was tinkering in my machine the other, actually stripping a few parts to diagnose my friends machine, when I put it back together I realised there is a molex connector on my motherboard (Gigabyte GA-MA790FX DS5) with something like PCIe written above it. Now I gather its some sort of auxillary connector for the motherboard (especially for crossfire). I havent plugged in a molex connector yet, but if I do, will it actually do onything on my single-card setup?
Also anyone know what the LEDs on Crucial Ballistix (2GB DDR2 800MHz) actually mean when they are flashing? Im gathering they mean its being accessed, but I heard it showed how full the RAM is or something.-(the system isnt actually broken)

Anyway if it helps. My specs
Silverstone Stryder 500W PSU
AMD Phenom 9500
AMD GeCube 3870 XT
WD 160GB SATA
WD 500GB SATA
WD 200GB eIDE
Antec 900 (Plus a Noctura side fan)
Asus 20x DVD-RW drive
Dlink Rangebooster 650N
2x 2GB DDR2 800MHz Kingmax
2x 2GB DDR2 800MHz Crucial Ballistix (with stock timings that a pretty good).

My older intel 915 board has the same thing. Weird thing was after two years of running a pcix card w/o the molex plugged in, one day the board would not post until I plugged it in.
My guess is it is auxiliary power --- doesn't hurt to plug it in. Like WR2 said, Check the mobo manual to be sure