6950 perform better that 4870x2. if i were you i will go for 6950 2GB and unlock the card to 6970. 4870x2 is equal to 5870 but if the game does not support CF you'll get a single 4870 performance only. other than that it will use more power and produce more heat than 6950. if you want to play dx11 titles in dx11 it can only be done with 6950. so what i'm saying here is that extra 120 is worth it if you can afford it
as i read its just a bios upgrade and your card will be axactly 6970 but there is a risk involved as i know. will send you a pm with link
that is why there are dual bios switch on the card. if you failed to flash the bios you can flip the switch and your card will be restored as 6950. i heard most 6950 2GB able to unlock into 6970 without much trouble. the only drawback getting 6970 form the unlocking method are you can't set the memory frequency to be the same as the original 6970. they said the reason is 6970 using different memory with better quality build hence the original 6970 memory can sustain higher frequencies than the one used in 6950
The $120 4870 x2 does run hot, but it is half the cost of the 6950. You would have to compare it with a 5830, 6850, geforce 460. I have a 4870, not the x2, and they are pretty fast cards. If you have the p/s to run it, it would be ok. Most early 4870's and x2's had to run underclocked to make them stable, so your looking at 700 core/850 mem, default is 750/900. It ends up being between a 4850 x2 and 4870 x2. Most newer games support crossfire so it should be fine.
At stock speeds the 4870 x2 is faster than the 5870 in games that support crossfire. The 5830 to 6970 cards are crossfire gpu's made on one die. amd just put 2 dies together. so games that do not support crossfire also only use half the shaders on the 5830-6970 cards as well. This is where the nvidia geforce has a advantage over amd. But it comes at a price of heat, power consumption and noise. (both the x2 and fermi).
If your power costs over 10 cents/kw/hr then get the newer power sipping card,
If your power is cheaper where you live than this card could save you some cash.
im using 4870x2 its a good card i even get good fps with all games around tho it was running very hot so i had to get a accelero xtreme cooler for 4870x2 now it runs 80C max with overclock i can play codboa at 56C* and downloaded crysis 2 playing it around 61-65C* im just writing my experience with 4870x2 so you can dicede what to do gl
Core clock 800
Memory clock 900
It is easy to do it and you need a software to do it. i'm using msi afterburner its a software helps you to change memory, core clock and voltage also fan speed. It also monitors temperature, gpu usage,speed etc. i used also riva tuner several years ago its also a good software. Also if you're gonna oc you'll prolly need to change your stock cooler to something better to not kill your card. Also you need to check the oc charts for more spesific info. I also don't know much at least more than this. Hope it helps
Rather than either, grab a 560 ti factory OC'ed for 10 extra bucks, or save that 10 and oc it yourself for free. (i chose to pay the 10 because i'm lazy) i recommend the MSI GeForce GTX 560 Twin Frozr II 1GB (250$)
to some what really matters are pure performance in game. overall 6950 is faster but as i said before the difference are small that you might notice the difference between the two in games. however its a different story if the game favor more on certain architecture provided by the gpu. that's why we see some game better with nvidia and some other with ATI/AMD