no power to cube -- logic board?

[mike]

Howdy.

I'm in a little bit of a fix, and I was wondering if I could get some 
advice from any of you.  I got a broken cube 500?mhz w/dvd for the low 
price of nothing, which didn't include applecare.  I'm a recent osx 
desktop convert from freebsd on x86, so I'm not yet comfortable with 
mac hardware, much less cube hardware.  an apple-certified tech told me 
I needed a new $900 logic board, so I might as well get a new iMac.  
Well, if I didn't want a cube, I suppose I'd go and do that.  Here's to 
hoping I don't get any advice like that on a cube-centric list. :-)

The cube gets power to the motherboard (green light goes on).  Fancy 
Dancy Electrostatic power button does nothing.  apple tech documents 
58780 and 88037 have been tried and failed.  Top assembly has been 
replaced but to no avail.  My thoughts are that I can replace it for 
$650, and get 800mhz speed at that ($300-ish logic board on the net + 
$400 800mhz powerlogix cpu upgrade/motherboard, unless I'm confused and 
the powerlogix upgrade includes the logic board -- I'm not sure if 
logic board and motherboard are different or synonymous), but a second 
opinion is much needed before I burn that kind of moolah.

Also, does anyone disagree with the apple tech regarding the logic 
board replacement?  Might my problem be the dc/dc converter, even 
though it provides the green light?  If so, does anyone know where I 
can get a replacement internal dc/dc converter, or about how much they 
cost?

Any help is much appreciated.

Mike