well, here's what I found online:

Whatever you do, don't forget to connect the ATX12V CPU power cable, as shown in Figure 16. If you fail to connect this cable, the system won't boot. When the Pentium 4 was first available, we forgot to connect this cable for the first half-dozen or so systems we built. Now, it's so automatic for us to do so, we search frantically for this connector on AMD Athlon XP and other systems that don't use it.


Connecting the ATX12V CPU power cable

I think I was using the wrong power supply, so I've switched to one with this cable coming out of the power supply... I'm connecting it now...

as far as using only 512mb RAM - I figured if it wasn't enough, I could buy one more for the other slot. As it sits, I'll be using 512 in one slot and 128 in the other... this is my first time building a machine, so hopefully it'll be a good basic one.

Any idea on how these wires should be connected to the motherboard?

'reset sw' = purple and white
'h.d.d led' = red and white
'power led' = green and white
's/b sw' = black and white
'speaker' = red and black