Hello there and thanks for answering my question. I have gone through two 4850s and with multiple OS reinstalls, driver cleans and driver versions I was not able to get past blue screens of death in my new computer. I am about to leave work and I am going to install a less powerful 8800 gt card. If the bluescreens continue, what else could I check and how would I do it? I have already tested the memory. I have a 520 watt power supply.

its possible its your power supply. 4850 in crossfire recomends a minimum of 550 watt psu.
450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
Even then i would personally go with something 600+watt cause alot of the time the psu really doesnt kick
out what is listed. if you still have problems it then could be your motherboard. any which way good luck.

From his reference to the single 8800GT, I'm guessing those 4850s were tried one at a time.
We need more information on your system. What mobo and RAM? If it is a P35 mobo with DDR2-1066 RAM, that could be the problem. With any RAM, it could be a bad stick (although you did say you tested it), or insufficient RAM voltage. RAM voltage generally defaults to 1.8v, and yours may need more. Particularly if you have >2 sticks, vdroop could be starving some of your RAM. Try with just one stick and see what happens.

He only had a single 4850 installed at any one time as jtt283 said.

Check your ram out like jtt283 said, and if you do happen to have another power supply; or a friend who might allow you to test your gpu in their system - check that as well

Assuming the computer does work with the 8800GT, but not a 4850, I'm going to guess that the PSU is not powerful enough. Since the make and model of PSU is not listed, it might not be of top quality. Even though it claims to be a 520wt, that doesn't mean its producing 520wts. Check the PSU tier lists and make sure there's a good PSU that is being used and not something from category 5. After that, perhaps a larger PSU, something in the 550wt-600wt range.

Yes, the ram may be suspect, but if it has been tested and it works with the 8800GT, I'd think it should work with a 4850 card, unless the PSU isn't putting out enough power.

First of all, the software that runs these forums is **** worthless. I typed my message out and tried to post it and got a message saying I must register. I hit back and my message is gone so I have to retype it. I proceed to log in and the only message I get is "Sorry, this section does not exist." Anyway I finally made it in so I will retype this.

The motherboard is a MB MSI P45 NEO-F 775 RT. The ram is MEM 2Gx2|ADATA ADQVD1B16K R . It calls for 1.9-2.1 volts. I had set it at 1.9 to start with. Later I raised it to 1.95 because of the bluescreens and it did nto help. The memory tested stable regardless. I gamed for a little over an hour today in bioshock and world of warcraft. The performance was obviously not as good as the 4850, but I did not get any bluescreens. I did get one soft crash, but no driver crash and the system remained stable. The powersupply is: PSU CORSAIR|CMPSU-520HX RT . I thought it was a good one when I bought it.

Well the computer is completely stable now, but barely faster than the computer I just sold. I basicly lost $500 for no gain. I am done with this **** for now and am trying to sell the rig on ebay for $500 which will lose me another $500. Hey oh!

Yes, that Corsair PSU should be a good one.
If you're giving up on it, there's not much point in further suggestions, but another possibility could be the sound drivers that the 4850 uses for its HDMI that the 8800GT will not have. Make sure they are cleaned up and/or disabled as well.