Processer Performance

If you want to run XP you will not notice any difference between a Dual & Quad Core CPU as XP only supports 2 Cores.

If you want to run a form of Windows and use all of a Quad Core you need to use Vista and not the bare bones version either. So if you are comparing Identical hardware except for the CPU here and are comparing a Dual Core on XP and a Quad Core on Vista then depending on the configuration of the hardware there may be a slight difference to a reasonable speed difference. The Quad Core on optimized Multi Threading Software will always be faster but as Vista is still working out Issues it may prove better to go with a Dual Core and a Proven OS then swap the CPU latter when Vista becomes more reliable. The Dual & Quad Cores made by Intel currently share the same Socket and Chip Set I'm not sure what the situation with the AMD ones are but with Intel CPU's it's just a matter of swapping the CPU when the price drops to a reasonable level to move to Quad Core CPU. I think that AMD is the same with the AM2 Socket but I'm not 100% sure of that.

Of course if you are considering using the Nix version of Adobe's CS3 then by all means use a Quad Core CPU you'll notice a massive difference in speed provided that you have enough RAM fitted.

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