Hardware change, Windows problem

Let me start off by saying that this wasn't the most intelligent way to go about things, but isn't a huge deal, so I didn't bother thinking too much about it beforehand.

I had a pc set up, and just recently replaced the hdd, mobo, cpu, and ram. The old parts were put back into their original case and hooked back up with the old psu and gpu (I had taken them out previously to put them in a better case and a better psu which are with my new components now). Before I changed out the old components for the new ones, I moved my important folders onto a second hdd (I had two spare hdds in the pc which are still being used by the new components). Then I installed Windows onto the new hdd. So basically, I still have the same mobo, cpu, ram, and hdd. The case and psu are different, but I don't think that will matter at all (or at least shouldn't, if I plugged everything in correctly), but the main difference is that I put the a different gpu in with the old components. I think that is what is causing my problem. I can't really remember, and it is too late for me to try it now, but I don't think that anything was shown through the gpu (and the mobo doesn't have a video output). My first thought was that it was a driver issue (went from a 7850 to a really old 8800 of some sort, not sure of the exact model though). I can't remember if the mobo splash still showed, and I will test for that later on, but for right now, what do you guys think? I would really like to get into the OS and sync firefox as well as mess with a few other things (I would like to keep the current Windows install, that is). I could try to take the 7850 out of my new build and put it back in there to see if that fixes it, but I would rather avoid doing that. I have some live CDs, so if that would be useful, let me know. Maybe I could manually install the nVidia drivers or something.

woah woah woah . . . 
So let me get this straight

  1. You copied stuff to a HDD, lets say HDD 1
  2. You then installed Windows on a new HDD lets say HDD 0
  3. You changed the GPU to a newer one
  4. Now you cannot get video output

If that is right, how did you install Windows without video output? if you used an older card, try using the older card to uninstall GPU driver than install the card.
Check if your GPU is getting enough power from the PSU and that you have attached any power leads it needs (6 pin etc) 

Put you 7850 back in and then uninstall the drivers for it. Clean the registry as best you can with Ccleaner or something else and then install your 8800.

That was what I thought would be the most straightforward and effective way to go about it. I was just hoping that someone knew a way to keep me from having to take the 7850 out of my new rig. Oh well, I guess.