Looking to upgrade from Q6600 DDR2

[attacus]

My current PC (stats below) does well to play nearly all of the games I throw at it (Crysis 2 & Witcher 2 at absolute max cause some framerate declines), but it's no surprise when my max resolution is 1440x900. The Q6600 is clocked at 3.4 and the 5850 has a slight overclock at 775. Things run fine on it, but I don't game as much as usual. It's mostly web browsing (but I do still need a powerful PC).

I was thinking of upgrading the motherboard + CPU + DDR3 ram. This ram http://www.amazon.co.uk/Corsair-CMZ8GX3M2A1600C9-1600MHz-Vengeance-Memory/dp/B004CRSM4I/ref=sr_1_1?ie=UTF8&qid=1344018220&sr=8-1 should be enough, I think.

The choice is of what motherboard and CPU to get. These 4 http://hothardware.com/Reviews/Z77-Motherboard-RoundUp-MSI-ASUS-Gigabyte-Intel/ and these two: http://www.amazon.co.uk/gp/product/B007KZQEHQ/ref=ox_sc_act_title_5?ie=UTF8&smid=A1WXV2FVMRQDT and http://www.amazon.co.uk/gp/product/B007KZQEGM/ref=ox_sc_act_title_3?ie=UTF8&smid=A3P5ROKL5A1OLE all look nice around the £150 mark.

They are all Z77 chipset for Ivy Bridge, but I was thinking of buying a Sandy Bridge chip, maybe buying Ivy if I found one for under £200. My question is is there a big performance gain using a Z77 chipset with Ivy rather than Sandy? How much functionality should I expect to lose (like PCI slots or PCI 3.0) by using a Sandy Bridge instead? And is there a clear upgrade path for the 1155 socket (are there future chips expected for it)? And would a PCI 2.0 graphics card notice any difference with a PCI 3.0 lane?

Secondly, what CPU has the best price-performance value? Is it better to skip Ivy Bridge altogether and wait for the 4th generation?

The most controversial question is whether it's worth upgrading now. I use the PC for a multitude of things but my biggest concern is performance of running any programs (not necessarily just games). I don't usually upgrade unless something's broken. But I would like to change to DDR3 ram while it's still cheap. I've considered swapping for a better motherboard (with crossfire in case I go down that route later) for LGA775, but it's not much of an upgrade.

 

[moornix]

I was thinking of upgrading the motherboard + CPU + DDR3 ram. This ram http://www.amazon.co.uk/Corsair-CM [...] 220&sr=8-1 should be enough, I think.

If you want to OC the CPU, you'll need a separate CPU cooler. Which also means you'll need low profile RAM.
Corsair Vengeance Low Profile (£40)
http://www.amazon.co.uk/Corsair-CML8GX3M2A1600C9-Vengeance-1600MHz-Memory/dp/B00569K7LM/ref=sr_1_1?s=computers&ie=UTF8&qid=1344020344&sr=1-1

They are all Z77 chipset for Ivy Bridge, but I was thinking of buying a Sandy Bridge chip, maybe buying Ivy if I found one for under £200.

The i5 3570k is good for gaming, but if you utilize video rendering programs or Adobe CS5, an i7-3770k is better.
i5 3570k (£175)
http://www.amazon.co.uk/Intel-Generation-i5-3570K-3-40GHz-Technology/dp/B007RUZKK6/ref=sr_1_1?s=computers&ie=UTF8&qid=1344019767&sr=1-1

The motherboards you mentioned are really just overpriced for their function. This one will do what you need:
AsRock z77 Extreme 4 (£101)
http://www.amazon.co.uk/Z77-Extreme4-Motherboard-Supports-CrossFireX/dp/B007KTY4A6/ref=sr_1_1?s=computers&ie=UTF8&qid=1344019822&sr=1-1

My question is is there a big performance gain using a Z77 chipset with Ivy rather than Sandy? How much functionality should I expect to lose (like PCI slots or PCI 3.0) by using a Sandy Bridge instead?

A Sandy can't use all the functions in a z77. An Ivy would enable the PCI 3.0 function whereas a Sandy will not. Since the Z77 is newer and thus somewhat more costly, a Sandy is better paired with a Z68.

And would a PCI 2.0 graphics card notice any difference with a PCI 3.0 lane?

No.

And is there a clear upgrade path for the 1155 socket (are there future chips expected for it)?

No, the Ivy Bridge is supposedly the last CPU type for the 1155 socket. The next chip, Haswell, is rumored to be a completely new socket type.

Secondly, what CPU has the best price-performance value? Is it better to skip Ivy Bridge altogether and wait for the 4th generation?

Right now for gaming an i5 3570k is the best performance for value. However, this does differ depending on what you use the system for. There will not be a 4th generation 1155 chip.

The most controversial question is whether it's worth upgrading now. I use the PC for a multitude of things but my biggest concern is performance of running any programs (not necessarily just games). I don't usually upgrade unless something's broken. But I would like to change to DDR3 ram while it's still cheap. I've considered swapping for a better motherboard (with crossfire in case I go down that route later) for LGA775, but it's not much of an upgrade.

That depends entirely on how badly you want to upgrade. As far as performance of the CPU goes, it really depends on the program. Media encoding/editing, and CAD generally require a fast multi-cored CPU. For normal users who don't do too much of that, an i5-3570k will be fine.