Is there a big performance difference between DDR2 800 and DDR2 667? Also, I'm not quite sure about the timings, such as 4-4-4-15; does lower timing mean better performance?
The difference between DDR2-800 and DDR2-667 is minimal. Maybe 3% at max. Still when building a nice PC with a C2D then I would go for DDR2-800.
Lower timings mean better performance yes, but from what I understand the higher they are the less chance of making errors. Either way the timings won't make a large difference.