The only chips coming out this year are going to be the extreme ones and cost £1000+ as far as i know.
No, the roadmaps indicate that Extreme AND Performance Mainstream should be released in Q4 2008. Performance Mainstream is where the Q9450 currently sits, the ~£200 price point. The Q9550 should replace the Q9450 at that price point very soon (Q3), and then the lowest of the Nehalems could be around that point too.
That said, all of these will be Bloomfield chips and require a LGA1333 socket motherboard, and yes, thats X58. Performance Mainstream parts can be paired up with either a Dual channel or Triple channel motherboard, and there is even rumors that DDR3, or DDR2 is up to the motherboard design rather than the processor. Its possible for the dual channel motherboards that different chipset will be needed. So it does seem possible that triple channel ddr3 will be the "first available" option.
The Quads (Even the Q6600 when it was first released) were all considered Performance Mainstream with the exception of the QX parts.
There is also likely to be several QX parts, with the highest part being around £1k inc vat, but there could well be lower parts comparible to the QX9650 which are still multi unlocked but lower priced than the very top model.
Nehalem isnt going to be an instant replacement for people buying low end dual cores (~£50), but for people buying a high end motherboard and a quadcore, it shouldnt be a huge leap to go Nehalem instead.
Of course Intel roadmaps are never 100% accurate, if there are yield issues, or a last minute "bug", then intel may only release the Extremes. And if yields are better than expected and Intel have a ton of chips to shift, they could lower the bar and release lower bin parts such as a 2.33ghz LGA1333 part.
As it stands, I would expect to see a 2.66Ghz, and 2.93Ghz "Q" part, and a ~3Ghz and ~3.2Ghz QX part released this year. Can I be wrong.. Of course, but I think there is a fairly good chance that the release will go "ok". Prices will still take some time to get to their proper levels though, The "£200" part might cost anywhere between £200 and £300 depending on yields.