They are very similar in performance, and both are very good. Don't anguish over any minute differences. If you have one predominate game you play, one might be better than the other, but in general, they are the same.
If you already have ati, buy ati, or, if you already have nvidia, buy nvidia. You won't have to change drivers, and installation will be easier.
the 4850X2 has seen some issues with game/drivers since I don't think it is an officially supported card. I personally would go with the 4870.
Not true! HD4850 x2 has been officially supported by ATI since cat 9.1, that is January of this year.
Please do not spread rumors if you dont own the card.
I have the hd4850 x2 1GB and I have zero issues. The only thing is that I have a HDTV and I game at 1900 x 1080 with everything at max so I need at least 1 GB of video ram
if you are to buy the HD 4850 x2 make sure you ge tthe 2GB version since it's 1GB per GPU or if you get two hd 4xxx in crossfire make sure you pick the 1 GB versions.
EDIT: Yes the HD4850 x2 is faster than the GTX260 and the GTX280, just search for some of the most recent benchmarks on the web using the 8.12 hot fix drivers or cat 9.1 drivers.
It does produce a lot of heat so make sure you have a proper cooling and air flow in your case. if you dont just go for the HIS 4870 1GB. You can crossfire it later as prices drop when ATI releases the 4890.
for $189.00 if you want better coolingyou can always get an after market cooler and oc the card to 800MHz or beyond for only $30 extra.
That's what I am doing.Check out the Acellero Twin Turbo Heatsink/Fan.
With this baby you can get the card up to 825-840MHz GPU