How far out can I go in the mining world?

I think I need to relocate my base in the mining world.

My question is: how far away from the spawn point can I go before it becomes a problem?

Also, what's this about too many chests causing server load? Do double chests count as two chests?

I've gone over 20k blocks on both X and Z, and have not been troublesome. But I would not recommend going out into miningworld for building your base, because I usually go around and raid stuff, so probably in a week your base will be destroyed xD. You could always go far out to collect materials and have your base in towny. That's what I do.

You could always go far out to collect materials and have your base in towny. That's what I do.

Can you suggest a way to go about doing that?