my sapphire HD6870 is currently sitting in x16, I am planning on getting another of the same card and crossfire.
However the other PCI is x8, will this cause a large decrease in performance? Thanks in advance for your help guys.
long answer is that in some circumstances and setups you could see like 5% performance loss and you'd have to look into how 6870 CF performs on your particular motherboard
Stock cooling is probably fine if you have a slot of space between the cards and decent case airflow (things you need anyways for crossfire to be a good idea) - you just won't be able to overclock much, if that matters to you.
I will probably have about an inch an a half of space between the cards. I am getting a new case with good air flow (Large fan at front small at back and a top exhaust) and psu at 750w. So I think im finally set!
6870 crossfire you will get microstuttering and 1gb is not enough to run modern games with high texture detail, you will get low performance. ive been there done that with crossfire 6850's. a month after i bought the second one i sold them both and went back to a better single card. i didnt believe all the other reports of microstuttering etc, and had to try it for myself. luckily i sold the card for the same price i bought it for on ebay so i had no reall loss, except time wasted stuffing around with radeonpro, and crossfire settings.