Unable to install Windows 7 64

I had previously been running Vista 32, however after reading up a bit (obviously not as much as I should have) I decided to upgrade my old machine with 4 gb of DDR2 (up from 2gb) and Win 7 64 bit. As can be seen in my system specs, my processor is a Pentium D 3.2 (840 i believe), and going on a thread a I read stating that a dual core can handle 64 bit I got that version. Install has been a nightmare, first it would give me an error that (according to forums) is a result of bad memory, tried everything with that (swapping out to my old memory that was fine, going from 4 to 2 gb... no dice) Next I took the drives out, and installed win 7 on them using a different machine (I have a much newer/nicer build as well, this is a second machine). Install went fine, so i moved the drives back to the other machine and I'm getting a BSOD on start up (and its a quick BSOD, cant even read it, flashes a second then reboots)... every time. I am a mouse click from ordering the 32 bit version but dont want to drop another $100 only to find out that maybe thats not my problem. Given that the machine was running fine with vista on it (even with the new memory) I am inclined to think this is not faulty hardware, just maybe incompatible software (the 64 bit version that is). Anyone able to give me a little more concrete insight?

Yes.

Memory has been tested, switched out with entirely different memory, reduced from 4 to 2 gb with the new memory... I'm pretty confident its not the memory. Win 7 is installed on the HDD in this machine but only because I moved the HDD to my other machine to install it then tried to move it back. I think the problem is that maybe my system cant run the 64 bit version but I purchased the disk OEM from newegg, and it doesnt have the 32 bit ver on it (so ill have to buy it again for $99). Just want to get some feedback before I do that, would hate to buy 32bit and have same crap happen.

I didnt think it would do that until I verified the version... i had the machine offline (internet) when I did the install... if not, my bad and that sucks lol. Either way I need to get another copy then and I'll probably go with the 32 bit to be safe unless you really dont think thats the issue???

Also, what would stop me from formatting the drive completely in the other machine, then trying a clean install again on the one im trying to upgrade?