does anyone know the difference in performance between two hd4850 @ X8 and two hd4850 @ X16?
is X8 enough? or will i need X16 to fully use the power of the two cards combined?

Can't say for sure, but I do remember seeing a Tom's article that the difference between x8 and x16 for PCI-E 1.0 on most cards showed little difference. I'd say that even with the advancement in cards, x8 vs x16 on PCI-E 2.0 would also produce little difference, especially since 2.0 has doubled bandwidth over 1.0, meaning 2.0's x8 is 1.0's x16.

8x will bottleneck the cards.

it will bottleneck it but slightly u wont lose that much performance.

The tests were on PCI 2.0 with a 3850 and 9800GX2, it won't bottleneck your cards for example Crysis was about 97% a 8x compared to 100% at 16x

But if you're a flight sim fan then 8x would be a sin

*edit* if what you have isn't PCI 2. then I'd suggest getting it

http://www.tomshardware.com/review [...] 15-11.html

http://www.tomshardware.com/review [...] 15-12.html

Since i have a DFI icfx3200 i would be very interested in more information regarding that bottleneck. Do you have any articles or reviews you base that statement on or is it personal experience?

http://www.tweaktown.com/articles/ [...] index.html

Tweaktown did a recent test between the P45 and X48 with 4850s and found that the 8x/8x was slower, by varying degrees, than the 16x/16x. Now, I take everything tweaktown says with a grain of salt, they aren't a great review site, but the test looked pretty well done. Check it out and see if the performance hit 8x gives is acceptable for you.

Very interesting. Thanks for posting it.

http://www.tomshardware.co.uk/foru [...] sfire-faqs

Dual 8x is a very good configuration too,it doesnt have alot of difference with Dual 16x and its cheaper too.
Chipests like Nvidia 750i SLI, Nvidia 650i SLI,NVIDIA 570i SLI, NVIDIA nForce4 SLI, ATI Radeon Xpress 200, Intel 975x and ONLY 2 Intel P35 boards(ASUS BLITZ FORMULA and ASUS BLITZ EXTREME) support Multi-GPU at dual 8x mode.

Nvidia 680i (Dual 16x) vs Nvidia 650i SLI(Dual 8x)
http://www.techspot.com/review/45- [...] -680i-sli/

8x will bottleneck them in a few games at high resolutions. Games like Crysis and Flight Simulator X, at resolutions 1920x1200 and above. Crysis gets bottlenecked a bit even at 1280x1024, so that might be a good indicator of how 8x is going to fare in the future when more demanding games come out.

But for almost all the games out right now, they won't be bottlenecked much if at all. You could run them on 8x then upgrade the motherboard in 6 months.