When you say " one of these new shimano or campag thingies" are you talking about cartridge BB or outboard bearings?
If it's a cartridge bearing you can fit either square taper or splined (assuming you have the appropriate cranks). It's an easy job - but make sure you know whether you have English threading or Italian and get the right kind of cartridge.
You also need to get the right spindle length or your chainline could be out.
Sheldon has all the details.
If it's the modern "outboard" bearings like the Ultra - Torque or the Shimano Hollowtech II then you don't need to worry about spindle length, but you "should" get the BB shell tapped & faced by an LBS before fitting.