I have waited long for SB but when i thought of purchasing i came to know p67 chipset has limitation of 8x8 for Crossfire.I have CF 6950 and my mainly concern for goin to SB is to CF as i have to use it for EYEFINITY on BC2.I am not sure but i suppose there should be lot of difference between 8x and 16x.I mean with p67 i wont be using full potential of CF,if yes ? then which combo should i go for ?Which cpu and mobo to look for?
You only lose a couple of FPS when using 8x slots, so a Crossfire setup won't be an issue.
If you really don't want to lose those FPS then you could look at the Gigabyte P67 UD7 which has a NF200 chip on it to bring you back up to 2 16x lanes, but it's a top-end board.
Pretty much any board made by Asus, Gigabyte or MSI will suit you very well, but make sure that the PCI-E lanes on the board are electrically 16x slots - some of teh cheaper Asus boards have 2 16x slots on them, but one of them is only electrically a 4x slot.
For the CPU, unless you are on a very tight budget then there is only 1 Sandy bridge CPU to choose for games: i5 2500K. You don't need the Hyperthreading that comes with the i7, but it's still an unlocked chip so you can overclock as much as you want.
If you don't want to overclock, then the standard i5 2500, or even i5 2400 will be just fine.
Very nice of you to clarify things above.but are there any mobos like ud7 with 2 16xlanes or its only ONE.as it is very expensive too.
One more thing i like to ask heard lot and read reviews too that for gaming i7 not need as it has hyperthreading.but what about if i have to use eyefinity means resolution of 5760x1920 on bc2,a game which requires more gpu power and along with few applications like Teamspeak and Steam also .And other gamin softwares also?Do i still dont need i7?
Nope, don't need Hyperthreading - the i5 is plenty fast enough to feed your graphics cards, and a 6950 Crossfire should be enough for your Eyefinity setup.
All Sandy Bridge chipsets will be limited on their PCI-E lanes, that's just the way they're designed. Only the expensive boards that use either a NF200 or Lucid chip can up the bandwidth, but as we said earlier, you only see a slight performance hit running at 8x.
Just make sure when you look at boards check they are Crossfire ready with the slots split 8x 8x, not 16x 4x
There is no difference between 2 16x lanes and 2 8x lanes because you can't saturate either of them. Benchmarks have shown that in games sometimes 8x wins and sometimes 16x wins. We are not talking 2-3 fps difference. We are talking about less than 1 fps difference.
The 2500k may actually be better than the 2600k for games at the same frequency because each it has 1 thread/core compared to 2 threads/core in the 2600K. Hyperthreading can be turned off in the 2600K. They both can be overclocked to the same frequencies.