It's best to match your RAM to your motherboard and processor. So, if you're getting a new Mobo and CPU soon you should decide on that before buying RAM.
If you've already got your sweet new mobo and CPU, then it's a fair bet some 800MHz might unleash some more overclocking power, but it depends on what you've got.
For stock standard use I think your 667MHz should be OK, but as I said, it depends on what you've got. A 'powerful gaming machine' needs 800MHz RAM, IMO.
Next step up is something like THIS which is 800MHz.
I don't know much about latency (except that less is more, or something like that)
Your motherboard manufacturer's website should list all the RAM it's tested to work in your mobo. A good place to start.