Where does this wire go?

I went to mini-box and bought some components I’m going to throw in to replace the dell optiplex sx260.

I ordered the following components. The total cost including shipping was just shy of $250.00

2GB DDR2 Memory
P4-12V to P4-12V Cable for M2-ATX
M2-ATX
Intel D945GCLF2D mini-ITX Atom 2×1.6Ghz
M350 Universal Mini-ITX enclosure

I’m curious to see how the dual atom does compared to the p4 2.8ghz of the dell.