I have the ATI Radeon 4870x2 graphics card and while gaming my computer restarts itself. It can take a few minutes to an hour game time for this to happen, depending on the game. I got my graphics card replaced with the same one and I still have the same issue. I then got a GTS 250 for testing and with the GTS 250 my PC no longer restarts itself. I can't figure out how to tell if this is a problem with my PSU (1000W), or if it was my computer overheating with the 4870x2 card. Also I tried putting the 4870x2's fans high, and eventually to 100%, and it still crashed my computer.
This is a well known issue with these cards. A lot of people point to the heatsink/fan assy not being able to cope with the heat coming off the gpu - even when running constantly at full rpm. You might want to look into getting a cooler for this card......
If you want to find out, I'd recommend that you do a stress test for your 4870x2.
You can download programs like Speedfan or HWmonitor and run a GPU intensive benchmark software like 3Dmark. Try to check if the temperature of your 4870x2 goes for more than 98degrees before restarting. If it does, then the problem is your video card.
I had a similar card. Well, not as high-end as your 4870x2. I used to have HIS 4850 IceQ 4 Turbo. When running at max, it runs at around 95 degrees but it runs perfectly. 48xx cards run really hot. The exhaust air feels like steam.
Alternative is to underclock your card and see if the problem goes away. If so, then open case and use a room fan to blow a lot of air on the card. If it runs at full frequency then you have a cooling problem.
GPU-Z (from tech power up, google it) will log the GPU temp to a file so you can see how hot it got before the crash. Nice card. good luck.
forgot to add, the gts250 (nvidia) uses a really different driver than the ATI Radeon 4870x2. If you have driver issues the card swap you did will not find them. Suggest you uninstall all video drivers and try the latest from ATI/AMD. Probably won't help, but it's a cheap and easy to try.