SB i7s is expected to be 10-20% faster clock-for-clock than current generation i7s in CPU heavy tasks. Also, they are expected to be clocked higher at stock and overclock higher using air cooling (and a multiplier unlocked chip). In games that make use of extra CPU grunt (GTA4, BFBC2, Civ 5 to name a few) then this will help to increase framerates, but with the majority of current games the graphics card will bottleneck performance instead of the CPU.
This is true - but for gaming the mainstream parts should be enough. You get the extra CPU power of sandy bridge plus dual channel memory and x8x8 PCIe multi-GPU support - ie
more than sufficient for gaming. It is true that the higher-end Intel parts are expected in Q3 2011 - but these are really only aimed at ppl who need crazy amounts of CPU power (for applications like rendering and number crunching), run multi-GPU rigs or like spending loads of money.
It does seem like a step backwards, but if they stay true to current prices the i5 K series (quad core) will cost less than £200 and probably the best bet for a gaming CPU. As for overclocking, intel has been showing off a sandy bridge clocked at 5GHz on air - so I reckon overclocking with a multiplier unlocked chip will be fine.
Even if you don't wait for SB, a current generation i7 rig should last you at least 2 years, so long as it stays coupled with a good graphics card. The main benefit with waiting for SB is that has more CPU grunt and will be able to keep up for a bit longer. But if you just want to get gaming now (and your current system is holding you back) - then I would go for s1156 i5 instead of i7 X58, due to the lower cost and no performance loss in games.
Edit: Please ignore all of the above (just say what current system you have) - if you already have a Q9450/X48 then please just keep it and overclock it. It is still a very fast quad core that plays games great and overclocks well - no need to upgrade that if you are mainly gaming. Get a new GPU (wait till december for AMD HD 6900 series) and marvel at the extra speed.