ok building new computer and its a bit expensive going with the 4890's, how much of a performance drop would i get if i want with the 4870's?
A decent amount, cause the 4870's doesn't scale as well for some reason
is it worth the extra $100?
I would say yes, but do it only if you have money to drop out of the window...I mean both of these cards are really good. If you have the money tho, go for it.
Depends what your wanting out of your system i personaly would go for the 4890
i agree with pamplemousse. If you have the money go for the 4890. It is definetly the better card. If you want to save about
$100, get the 4870x2. There will be a noticeable performance gap between the two cards. The 4890 beats the 4870 in all the main areas
The 4870X2 isn't that great, it scales so bad that the 4850X2 is only behind by a few frames. The 4890X2 is likely to outperform the GTX 295 at release. Not only that but the 4890 is a more power efficient card and can overclock a lot more than the 4870.
It is worth an extra $100 I would say.
ok, cool... now can someone explain the difference between a 4870x2 and two 4870's... is there any performance difference???
umm i don't think there's gonna be a 4890x2?... 
The performance difference between the 4890 and 4870 is roughly 10%.
Since 2 4890's scale very well at nearly 50%, the assumption can be made that a 4890 Crossfire X setup will be roughly 20% faster.
However, your resolution will make a huge difference between the 4870 and 4890. I would say less than 1920x1200, there is almost no difference (in real world application). The larger the resolution, the bigger the differences will become, because the GPU will be working harder.
Keep in mind though, You can always overclock 2 4870's and get very close to the performance of the 4890's.