IMO sleeve over coilovers are worthless unless you plan on purchasing a good set of aftermarket shocks too.
However..... If your going to buy coilovers and shocks, you mine as well just go with a Full Coilover setup.

Then again, If you do want to get coilovers and you just use the car for everyday driving, Then IMO your wasting your money and you should just buy a set of springs.

Sounds like a vicious circle doesn't it?

So, How Low do you want to go, Do you plan on Auto-Xing or just using the car as a DD?
What is your budget?