Either the GTX580 or GTX570 is what will let you play those games on Ultra settings.Although Witcher 2 is just as CPU demanding as it is GPU.You will need a powerful CPU for that game.
I think the GTX570 has probably the best cost to performance ratio.While it doesn't cost $500 like the GTX580 it can compete with the 6970 but for less.The cheapest GTX570 I can find as of now is a MSI reference version that goes for $290 after MIR and their is a Galaxy that goes for $260 after MIR
I wasn't justifying the cost I was just saying in order to play BF3 on Ultra settings and maintain 60fps you would need a GTX580.I agree tho the GTX570 for $320 and $260 after MIR is probably the best bang for your buck on the market right now.
Well I don't know to much about Intel motherboards but what I read is that the Z chipsets are more geared towards Intel Smart Response which for use with a SSD.I thought their was more of a difference between the chipsets but a quick search revealed this.
p67 does not support on cpu video
z68 supports on cpu video, plus intel smart response
z68 is the chipset to get along with an SSD for caching.
So their's not much of a difference between them.So go with either one is cheaper.
As for the 2500k I don't understand what you mean.Your tired of O.C.ing?I say it's worth the money because it's fairly easy to O.C. since it's an unlocked multiplier.And you never know in a year or more you might need more performance and you can just O.C. easily.