I have been considering this for quite a while now, and I think it is time to start looking into what to do about my problem:
At the moment, my system is running 6GB DDR2 RAM (2GB of it from late 2007, and the other 4GB from late 2008) and an Intel Core 2 Quad Q9550 (Since September 2009).
I have been researching into DDR3 memory, and I have seen the difference in speed over DDR2, however the number of motherboards within a decent price range, from well known manufactures and with an LGA775 port are very small, so I think it is time to upgrade to an i5/i7 processor (I have heard they are much faster than the older Core 2 generation).
I have been looking into the Core i5, 2500k which has a clock speed of 3.33GHz.
How much different is this over my current Q9550, as I notice the Q9550 has 12MB L2 cache, but the 2500k doesn't have any?
Also, are the benefits I would get from upgrading be worth around £300? (I will be grabbing a new motherboard and 8GB DDR3 RAM clocked at 1666MHz).
Will it be? hard to say. It will greatly depend on what you're doing. Your general user experience will probably be smoother, but the difference won't be very noticeable until the CPU is really pushed. Thus, it would be more worth it it in workstation tasks than light gaming.
To put it in perspective for you, ANANDtech has an archive of benchmarks they update and can help you with this decision. There's some instances where the i5 2500k is nearly twice as fast as the C2 9550.
Ivy Bridge is not expected to be released for sale until March 2012. So if you can wait 4 months for benchmarks, then that would be the best option. The Q9550 is still a very capable CPU, especially if you overclock it. It is on par with Phenom II and Bulldozer CPUs. I'm still satisfied with my Q9450 OC'ed to 3.0GHz.
Current estimates of performance improvement (same clock speeds) range between 6% and 10% over Sandy Bridge. I am leaning towards 6% because Ivy Bridge will just tweak an already mature architecture core for better performance. Most of Intel's efforts is going towards Intel's next gen CPU; Haswell. The Ivy Bridge's die shrink should help boost clock speeds though.
In terms of pricing, I would say IB CPUs should be released at the same price points as how the SB CPUs are currently priced.
To answer your L2 cache question, the i5-2500k has 6MB cache.