Which Core2 Quad is it? They made several at 2.66 GHz.
All of the Core2 Quads at that speed will outperform the i3 540. They're also quad core, while the i3 is dual core (I don't remember if the i3 has Hyperthreading - if it does, it will be two physical cores + two virtual cores).
The real question is whether or not you'll notice a performance difference. That depends on your actual usage. Normally I'd say go for the Core2 Quad, but that's an older CPU and likely on older hardware -- that means you may be more prone to failure. If you aren't concerned about the possibility of restoring backups or rebuilding your hardware, the Core2 Quad is the way to go.
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I agree with @FRH_Lisa, it all depends on your usage. If most of your applications are single threaded the higher clock speed of the i3 could lead to better overall performance, with the reverse being true if your usage is heavily mutli-threaded.