Here is a tough one for you.

Computer:
Homebuilt, running XP Pro, two HDD's (one WD 120Gb, one 40Gb), one DVD ROM, one CD-RW, one floppy (yes, I actually have one), SB Live, Generic 56k Modem, and a 128 Mb Gforce video, 1Gb RAM (two 512 Mb sticks)

Problem:
PS powers up, fans run, HDD's spin up (I belive), no POST, no beeps, no continuing.

Before computer died, had odd failures from USB ports.

Replaced so far:
MB & CPU

RAM tested good, PS has good voltages.

I need help, this has been down for over a month, and no one is able to help me.

If you power on without any RAM does it beep?

no beepie.

Assuming that the motherboard (or case) has a working speaker that would indicate a bad motherboard or bad CPU or an incompatibility between the two.

Do you have both the ATX 24 pin Main Power connected as well as the 8 (or 4) pin +12V connection connected?

Yep, I got the new CPU and matched it to the MB. The 24-pin main plug is well set, and the four pin is set also. The MB is new, along with the CPU.

What I have replaced in the computer hasn't changed how the computer is acting. Should I try replacing the PS? I don't think that the PS is the problem, even though the PS is putting out the correct (within tolerances) voltages. Is it possible that the current isn't enough? It is a 480W PS, and should have MORE than enough wattage and current for what hardware is plugged into it.

480W should handle most systems. There is also a minimum end on some PSU's. In other words: "Are you drawing enough Watts?" would be the question.

Do you know that either the board itself or the case has a working speaker?

Are you sure that your RAM voltage matches the motherboard's?

Interesting question about the minimum wattage.... I will have to try to look into that one.

MB should have a working speaker.... moved it over from the other MB, and didn't have any probs with it there. I wasn't sure if it was plugged in backwards, so even tried it opposite from what I had it on.

RAM should match up. I got a replacement with the same specs, aside from USB2.0 vs. 1.x, compared to what the previous MB was. I tried to get everything the same.

Some newer boards have an initial RAM voltage set to 1.8V whereas a lot of RAM (old & new) are set to run at 2.1V

With 2.1V RAM in a new 1.8V board nothing will happen. In this case one would need to install a 1.8V "stick" to enter the BIOS and change the mobo voltage to 2.1V

This is just my last learning experience with my last build so I thought I'd mention it.

If I think of anything else I will pass it on.

To be honest, I am not sure the voltage of the RAM...... or what is set on the new MB.

One stick is a PC2700 DDR 512Mb, the other is PC2100 DDR 512Mb. I don't see anything on them that states voltages....

This has really been bothering me for a while, and I appreciate all the help that you have given.

If you know the specific make and model of the RAM and Mobo this info can usually be found on the manufacturer's web site.