What you need: 2 atx psu's, some wire, and space in the case (may have to remove the drive bays in the back if you have them)
Setting up your system to run dual psu's is very simple. First thing to do is make sure that both psu's work. Now locate the plugs that hook up to the motherboard on both psu's.
Link wire 15 (black) on both psu's together, line 15 is ground and to run dual psu's you need a common ground.
Link wire 14 (green) on both psu's together, line 14 is the "ps-on" line which turns the psu's on. Now your done with the master psu, the master psu is the supply that plugs into the motherboard.
Always make sure you have a load on both supplies, even just a little floppy drive will do for test.
Atx psu pin out, note that other supply's may use different wire colors.
Just make sure you have some kind of a load on the second PSU, some won't start if they don't. Personally I would go with the relay method but its so costly that i just get a bigger pcu supply
how it should be hooked
amd athlon xp 2400+
1 apacer 256mb unb pc2700 cl2.5
motherboard msi kt4av via kt400a chipset based
1 ??? 512 mb pc2700
1 agp 8x/4x (geforce 4 predator mx4408x)
2 year old 21 magnavox monitor
1 aquos sharp 15\"tv
Seagate 40gb hd
DC Output: 420Watts Total: +5V/0~42A, +12V/0~18A
+3.3V/0~26A, -5V/0~0.8A, -12V/0~1.0A, +5VSB/0~2.5A.