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8% difference in speed, not worth it. However, since those are multi-core aware apps, perhaps an i5 machine might be in order?
 
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.
 
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