There will be no big difference clock for clock, i.e. with a dual core application, like most games, that quad at say 3ghz will not be much better than that dual at 3ghz. On the plus side the q9550 is 45nm, meaning more lower heat and power consumption.
Far cry 2 uses 3 cores, so i suppose you'll see some difference there. I have to say though, that for gaming, there is no need to upgrade from what you currently have, not yet at least.
By the time you really need to upgrade, you will probably end up upgrading more than just the cpu; maybe the motherboard and ram as well. But what you currently have will certainly suffice.
Even with i7's available, i still don't think you really need to upgrade. Maybe if you were starting from scratch, then it could justify the i7 build. But an i7 920 will not give a great boost in gaming when compared to even an E6700, assuming the E6700 has a decent overclock. Certainly in other applications, especially quad core enabled applications, it would be a worthy upgrade, but for gaming, no, especially considering the price.
Perhaps if you were selling your old system, or at least the old parts you intend to replace, then at least it can take some of the load off the price for such an upgrade.
wait til i5 comes out, then you will probably see an i7 price drop. and It should only come out to £500 even now anyway because there's some g.skill 3x2GB 1333mhz ram for £45 on overclockers.co.uk so maybe try there but wait for the price cuts and it should take the price down to £450 after the price cuts.