Well I've a wide board of GPU options for the next month about mid builds, so i have this doubt bout ATI and Nvidia GPU comparison.
I already know that ATI's HD4850 outperforms even the 9800gtx and some says it might be a better option against the 9800gtx+ for some cost reasons, so my doubt starts when i want to decide upon ATI's GPUs; is the 4850 still better than the 3870x2 ?? if i want to Crossfire a couple of them, which would be better to do so?? and if the 4850 really outperforms the 9800gtx and the top GPUs of the 8000 series, why it costs -200$ while Nvidia's ones cost above 200$ ??

Oh no its all true the ATI cards are cheaper to make for a variety of reasons. Firstly they are getting great yields on there chip wafers. basically the chips are made in a giant wafer, i don't know specifics though some on here do. The chips on these wafers are subject to a certain amount of variation as its not a perfect process. All the chips are tested and a percentage make the desired grade and some don't. This is how you get XT/PRO/GT/GS/SE and so on cards from.
The 4 series chips are getting a very high percentage of good chips, this means they can sell more of the wafer for the full cost instead of having to sell them off cheaper as lower spec cards. Obviously this allows them to lower the price.
Also the chips are 55nm and the Nvidia ones are 65nm so it takes less actual silicon taken to make them.
Nvidia are said to be getting bad yields so end up having to charge more to get the same revenue from a wafer.
Hope that helps.
Also technically the 4 series is far better than the 3 series as it handles AA a lot better among other things, like having 800 sp's instead of 320.
Toms have a couple of reviews if you look under articles and reviews you can find out all about the new tec of the 4 series.
Mactronix

Mactronix...thanks for the explanation about yields and wafers. I know that it's not super technical, but was really helpful in understanding the basics.

I learned something here. Thanks.

Glad I could help any more questions don't hesitate to ask, the people on here are nothing if not helpful.
I have not stopped learning ever since I started posting.

Mactronix

When CF works for a game, the x2 is faster. If my memory serves, the 4850 is a better card overall however, as CF doesn't work really well for most games. Remember then when CF doesn't work, you only have the power of a single 3870. Even if CF does work, not all games see boasts of 50% plus. Meaning the 4850 is still faster. Add in that the cheapest price for the x2 is around $270 after rebates and shipping, while the 4850 is down to a mere $175 after rebates and shipping, I vote for the 4850. $100 cheaper, and more consistent performance.

On another note...

I have a nForce SLI mobo, and was wondering if the ATI cards will run AS GOOD on that as it will on a xFire mobo (single card configuration, naturally)?

Yup, it should.