Despite the higher cache on the 3.33 Ghz processor, benchmarks still hover around an eight per cent improvement compared to the 3.06 unit (see here for Geekbench 2 results across the current range of iMacs and Mac Pros). That's fairly modest for an extra £160 (UK).
In the higher models, the jump from an i5 to i7, or from the 3.06 (27") to i5 brings you a lot more processing power for your money.
If it was me, I'm not sure I'd be willing to pay a premium for the negligible gain that a 3.33 Ghz processor gives you over the standard configuration.
On the lower models I'd be tempted to put that £160 towards more RAM or better still move up to the ATI4670 card.