I'm building a new PC and wanted to know whether having the full dual @x16 on the AMD 890FX chipsets will have much of a difference over the dual @x8/x8 on the p55 chipset. I was thinking of either the Phenom II 965 or the i5-750, and also wanted to know if there will be any SLi certified AM3 mobos (because I love the solid scalability of Nvidia). Also, 2 5850's in Xfire or 2 GTX 460's (1gb) in SLi. Which would be the best?
16x/16x doesn't really matter. It's only 4% difference, and only comes into play with dual 5970s.
That said, the 8xx chipsets aren't worth it anyway. You pay more, but only get the ability to drop in a X6 CPU (which you won't need) without doing a BIOS update (which is free). As for exact boards, I'd look at the Gigabyte GA-790XTA-UD4 for the X4 955 (the 965 is just a factory overclocked 955) or the Asus P7P55D-E Pro for the i5-750.
For the GPU, the 5850 is better performing in most games. I'd still check the exact games you're using. I willl also say that not much is thought of nVidia's new cards, as they tend to be expensive, under perform, run hot and use a lot of power.
I'll also point out that you're better off starting a build with a single massive GPU (like the HD 5970 for instance) than two weaker cards. The bigger GPU might not be as powerful or might be more expensive, but you keep an upgrade path open.
I was reading reviews on the GTX 460 SLi and the 768mb 192bit GDDR5 catches up to, and most often beats, the 5850 in Xfire. The GTX 460 that i will be getting is 1gb adn 256bit GDDR5, so i imagine that it will scale much more effectively and beat the 5850 in most games. Also the GTX 460 is about AUD$50 cheaper than the 5850. The GTX 460's have also shown very good temps and noise levels, less than a 5850. Also i cant really afford a 5970 cus AUD$800 is a bit steep for my budget, because im buying one card first, then a second card later
No, the 768 MB one beats the 5830, not the 5850. There's a big difference between those cards.
I wasn't suggesting that you get the 5970 specifically, just that you should buy the largest GPU you can afford. If you can't get the 5970, the 5870 is the next best thing. After that, it's the 5850 followed then by the GTX 460.