I have an eMachines T3304 Desktop I bought brand new in Feb. 2006. At first, I was having problems getting it to come on. Sometimes I would have to restart it with the POWER button like 10 times before it would ever come on. Sometimes I would have to unplug it for 5-10 MINS & then it would come on. 2 days ago I went to turn it on & the blue power button light come's on & stays on, & the fans are running, but nothing at all on the monitor. The little amber light under the power button isn't coming on, or flickering like it usually does. Today, I pulled off the casing, & all fans are working & no obvious loose or burnt wires or connections. A friend has a similar eMachines that runs fine, so I took mine over there. I swapped the Power Supply, Hard Disk, Video Card/Expansion Card, still nothing. I disconnected all hardware & peripherals, reinstalled one at a time checking each time to see if it helped, & still nothing. I'm STARTING to think it's the Mother Board. Does anyone know anything else I might could try ? Is there something on the Mother Board I could check, or something else somewhere in the computer ? Any Help would be GREATLY appreciated. Thanks ! Reading your post, I'm starting to agree with you, for several reasons:

1) You've tried everything else that could conceivably cause this issue

2) Failing motherboards can do that.

3) eMachines suck <-- My biggest reason!

I've owned a T2682 for years. I hate it! It literally fell apart around me. From the failing DVD-ROM to the mouse turning green, the thing just barely survives in my sister's bedroom now.

eMachines uses sub-standard hardware so they can sell cheap. This isn't a suggestion I normally give, but I'd suggest getting another computer when it's financially feasible. I promise you, the cost you will spend on replacements and repairs will dwarf the initial cost of purchase. I've spent over $300 on my $400 eMachine, just trying to keep it alive. I actually kinda like eMachines, but mostly because they're decent computers...for that price. eMachines are more like starter computers. As much as I liked mine, I can't really argue with Dilbert because mine crapped out just like yours. In both of our cases, I think it's a motherboard problem. You could try replacing it, but it's like an old car: you'll just have to keep spending money on it to replace parts when you should be saving up for a new computer. Computers aren't cheap, I know, but you would be better off with a new one. Then you could slave your eMachines hard drive and put the RAM in your new computer. It's not much of a boost, but it's better than throwing them away.

But yeah, it's entirely up to you. You can try a new motherboard until you're able to afford a new computer, but keep in mind that other things will start failing soon. Your CD drive is probably next, if it hasn't already GONE south.EMachines are known to have PSU issues...you are testing it with another EMachines PSU.
I'm not convinced it's the MBoard yet...You can get any type/manuf board as long as the slot fits your CPU type....i'd buy a new PSU while i was at it as well...