No Signal to monitor - please help

Firstly, this is my first post on Tom's Hardware, but I have used this as a most valuable resource for quite a few years. So I would like to say thanks in advance for any help that anyone can provide. Also, I apologize if I have posted this in the wrong section, but since I don't know exactly what the problem is I didn't really know where to go.

Here's my issue: I just purchased new parts for my current PC, and I sold my old parts, which were only a little over a year old, to a friend of mine. Here are the parts that were sold to him: EVGA 8800 GTX, Foxconn C51XEM2AA- 8EKRS2H, 2 2GB Sticks Kingston HyperX, AMD 6400+. All of these parts were working flawlessly for me just a few days ago.

Along with these parts he purchased a 850w Sevensteam Power Supply, and the hard drive that he is using is from the previous computer that he owned, and I've verified that it is working by plugging it into my computer.

After migrating everything over to his case and turning the power on there was no signal to the monitor. Although there is no display, the lights on the motherboard (two green and two yellow) light up, and the digital display shows to dashes. All of the fans work, I hear the HDD on, the DVD drive functions, and both the CPU fan and 8800 GTX fan works.

Here's what I've done to try to narrow down the issue:

> Plugged the 8800 GTX into my motherboard -> it worked fine
> Plugged my 5850 into his motherboard -> when I turned the computer on the cards fan starting spinning at 100%, whereas it was barely noticeable when on my motherboard
> Disconnected all non-important parts, but still no POST
> I took out the RAM and only used one stick in each slot, but still no luck

I am about to pull my hair out trying to fix this problem, because he purchased the parts from me and now it seems as if I'm going to have to refund him with money that I don't have.

If anyone has even the slightest idea as to why this is happening, I would greatly appreciate a reply. I'm hoping that it's not a bad processor or motherboard, although it seems that that's what everything is pointing to.

Again, thanks in advance!