Hi
 
I was starting out with a new machine and the ASUS Maximus Formula motherboard, and an ASUS 8800GT, 2GB RAM and Core 2 Quad Q6600.  This worked fine, until I desided to overclock, by SLI, which , obviously would not work with the Intel X38 chipset, on the motherboard.
 
I have now switched the motherboard for an ASUS P5N-T deluxe, and added the IFX14 cooler with 2x120mm fans including the extra Hr-10 +80mm fan, for the backside of the motherboard.  
 
However, I am not able to start up the machine!    It boots correctly but will not start Vista.  Tried to reinstall (several times), but it freezes the screen at some point during the installation, never the same place though.  Have had up to 44% unpacked files, but also the short version where it stopped before the "rolling green stripe" had finished.  
 
I have not put in both video card during this start up, and also I tried to install a new, untouched, hard disk, but no luck.  
 
Could it be overheating problem? ......or am I lacking power from the power supply?    
 
The power supply is a 500W Artic cooling, stating 350W on one group and 140W on the next group.  It does seam a little low (as it was purchased to be a 500W), but will this act like that during a simple installation of Vista or XP?  
 
I really need an input of which way to go.

I'm not sure what you're talking about in your 1st paragraph: you have 1 8800GT & you're sli'ing it? How?
 
500W is alright for a stock quad + 8800Gt pc. 425W minimum. Overclocking requires more power. My suggestion is clear cmos & run the pc at stock. If it is unstable at stock, why bother overclocking it?

Isn't the x38 a crossfire chipset?

Make sure that the memory is getting the correct voltage. Try installing with only 1 stick of memory, then add the rest after installation.  
FYI, overclock3D.net gave this board only mediocore ratings, and not a good board at all for overclocking, while providing sub-par 3D gaming performance. In short, they were not impressed with it.  
They may of course have just got a bad board, and maybe some of the issues have been fixed with BIOS updates, but it's kind of a bad omen right away for what you are planning with the new board.

Sorry if I am unclear, but I tried to simplify, which is not good when describing.
 
Case is:  I bought a second 8800GT card, and installed this on the Maximus motherboard.  Obviously it would not work as the X38 is the Intel chipset, which runs the ATI Crossfire.  This is the reason for changing the motherboard for the P5N-T deluxe, with the nVidia chipset, which supports the SLI mode.
 
It is unfortunate that I did not see this, but what I learned about this before I purchased, was that it was the new chiset from nVidia, and I was kind of hoping it would work.....
 
I will now try with a single block of RAM and check voltage.  If it won't work I will clear CMOS, and retry.....
 
....this must come to work............
 
Any ideas of the power demand, WHEN I get it running and does put on the second video card, to run SLI mode ?
 
When it crashes the heatpipes of the cooler system are very cold

Does the bios revision the new MB come with support your CPU?

The MB does support the Duo and Quad processors.  But how would I switch the bios version without control of the machine?
 
Next try will be clering the CMOS as "akhilles" suggested.  How much does this clear?  the manual indicates the time and location, beside of this, is it more than a reset to default in the BIOS ?  

The manual is usually wrong about clearing cmos. Power off, disconnect power cord, remove the battery, put jumper cap on clear position, wait 5-10 mins, install battery, return jumper to normal.
 
It'll clear everything to factory default. Date & time too. Usually back to when the bios was released.