screws and PSU ??

I'm putting together my first system and there is no info in the case manual about which screws to use for the mobo. Does it matter as long as they fit? Should I use the same ones as the ones that mount the HD and DVD to their brackets? (Well, the HD screws are different, but that's for the sound dampening bracket).

Also there are 2 white plastic screws and I have no idea what they are for.

I am using the Antec Sonata case now (for a friend) and will use the Antec P180B for myself when my mobo comes in.

As for the power cables, from the front panel there is a cable with a male and female plug on the same cable. I figure one gets plugged into the PSU, but what about the other one? Does that go on the mobo?

Thanks!