Q: Should I get a 3GHz Intel Core2 Duo, or a 2.66GHz Intel Quad Core? I'll mainly be modding/playing Crysis. So which one?
A: Very few apps/games are currently written to support multiple threads, and most of the ones that are only support two threads. Recent high-end games like Crysis might support 4 threads, I'm not sure, but as it stands right now you will almost always get better performance from a faster dual-core than you will from a slower quad-core. The only major exceptions are 3D rendering and audio & video conversion. If you want to be prepared for the future when developers start adding multi-thread support to games and apps, go for the quad-core. Provided you have a decent heatsink/fan and a case with good airflow you should be able to overclock the quad-core up to around 3 GHz on stock voltage. If you have options to increase the voltage in your BIOS you can probably get it up to around 3.3 - 3.5 GHz. You could probably get the dual-core close to 4 GHz, though. Of course none of this matters if you don't have a powerful video card.