Timings

Alright, so I have a 3 modules that are a year or 2 old that I kept from my last system, would prefer others, but I'm not going to fix something that isn't broke.

OCZ Obsidian rated at 1600MHZ 9-9-9-24 1.65v.

I have been overclocking an i7 930, and currently have the CPU at 4.0 and the memory set at 1536 MHz with the timings at 8-8-8-21, 1.65v.
I don't have the best knowledge when it comes to memory, so I have a few questions:

- 8-8-8-21 / 1536MHz is generally preferable over 9-9-9-24 / 1600MHz, no? I figured the tighter timings with a slightly underclocked frequency would give better performance than looser timings and a tad more freq. The tighter timings don't work at 1600MHz

- Additionally, I hear these modules are sensitive to tRAS settings, but that I have seen people that have the modules at 7-7-7-24...
I have only tried 7-7-7-21. EDIT: 7-7-7-24 boots but errors.
Is 7-7-7-24 more desirable than 8-8-8-21?

I understand the basic concept of timings and latency, but I don't know about ratios and if they're important.
For example: Why is 7-8-7- more commonly seen than a different combination like 8-7-8- ?
Is 7-7-7-24 a negligible difference from 7-7-7-21?
Is there a "proper" ratio between the 4 settings?... or is it just: "See how low you can go on everything, and that's the best."

So, I'm wondering if there is a general rule or guideline for the different values... are there any well known "No-No's"?
Or should I just mess around with it and not worry about ratios.

Thanks everyone.