CPU POST test fails w/ my 520 and 420 PSU but not with my 300. After switching on and off about 3 times it works with my 420 and 520 until I turn it off for a little while.

Your 300 watt PSU may have more current output on a critical voltage.  Post what the current rating is for 12v, 5v, and 3.3v for all the units.  Your problem sounds like a power supply issue.  You can also try reducing the load at startup by disconnecting drives and cards and see if that affects the boot process.

I agree totally with Callandor.  The most important is not the wattage, but the voltage....if you can post your voltages, we can go on from there.

Just to give you an idea, At my store, we have some cases that come with 450watt PSU's...that are reallllyyy cheap.  One of the most important parts of the Power supply is the voltages.  The 450watt PSU's (cheap ones) the 12volt rail has 15Amps.  the 5v has like 32Amps, and the 3V has 38Amps.  I have an enermax 365watt PSU (this psu generally cost around 50 bux in most computer stores....) The voltage rails on my power supply is as follows:12v=26A 5v=32A and 3v=32A.

The 450 watt pSU was not enough to power up my system which is only an AMD 2100, 1gig of ram, but I have 6 Hard drives, a SCSI Card, TV Tuner card, Radeon 9700 Pro, a DVD rom drive, Cd burner, and 6 case fans, and an alpha Heatsink w/ 80mm fan.  however my 365 watt PSU works great.  However if i add one more fan, my system will nost post with the 365watt either.  But that's how good my PSU is....
In order for the 450 watt PSU to power my system, I have to either take off at least 4 of the fans, and 4 hard drives...and can power up the rest.

-Ok
heres all the stuff i have plugged in (other than what i mentioned before)

-Lite On 52x32x52 CD writer
-Lite On 16x DVD reader
-Mitsumi 3.5 inch floppy drive
-D-link 10/100 NIC
-Linksys 10/100 NIC
-3 Com 56K modem
-Maxtor 80GB SATA Hard Drive

Here are the voltages on my Power Supplies--
420 watt Raidmax --
3.3v=26 Amps
5v=32 Amps
12v =13 Amps

300 watt Delta Electronics--
12v=15 Amps
5v=30 Amps
3.3v=26 Amps

520 Watt Raidmax--
3.3v=30 Amps
5v=32 Amps
12v=35 Amps

Rails seem steady on all of them but on my 520 and 420 PSU the 12v rail is like 11.85 Volts. I haven't tested my 300 watt.

Thanks
--Ian

Your 520 PSU looks better than your 300, but why it doesn't work seems strange.  I suggest stripping the system down, and adding components until it stops booting.  Start with the video card, one stick of RAM, the cpu, and reset the BIOS.  Then add RAM, add the hard disk, etc.

Already tried that. Still fails when I try with just the motherboard and CPU, unless im using my 300 watt. Im starting to think somthing is wrong with my motherboard. Also tried with just 1 stick of ram motherboasrd CPU and video card and still nothing unless i use my 300 watt. My 520 works, just after i switch it off and on a couple times, same for my 420. Im using my 520 right now.

--thanks

Your 300 is behaving normally, and I would be more inclined to think the Raidmax supplies are acting strange.  Two of them at the same time does seem odd - do you have another non-Raidmax brand to try.  My thinking is that if it is the motherboard, the 300 should have been a problem, and more so, because it is rated lower.  There is a common thread in the brand chosen, so try to rule that out.

Sorry.
my 520 watt is an Aspire
Just messed up typing.
I have tried all 3 of them with my other system and they work
Here is my other system--
P4 1.5A CPU
Dont know the motherboard
256MB kingston Value RAM
NEC 16x DVD-ROM
NEC 12x8x42x CD Writer
Floppy Drive
40GB IDE Hard Drive (Dont know the brand off hand)

--Thanks