A8N SLi Mobo Problem.

[Napster]

My friend recently bought a new computer:
Specs: A8N Sli Mobo 939
AMD 3800+
1GB of Corsair DDR 400
200 GB Maxtor IDE HD
(2) 6800 GT's
500 Watt Aspire PSU
X-navigator Case (aspire)

And for some reason we can't get it all together ()
The power thing is a BTX, and the PSU is an ATX (look at picture)
it still fits inside but its a 20-prong PSU cable (male) into a 24-prong mobo female connector, it still fits inside but leaves open 4 prongs. Is it backwards
compatible? does this make a difference?

[Ebaum]

we got it nm