Hmmmm well the heater does take its heat from the coolant, so it sounds like there is a blockage somewhere, otherwise your coolant pump (water pump) isn't working, but you said you replaced that so it should be working.
Ummmmmmm, it's interesting that the heater is blowing cold air, I think that will be part of the problem of the overheating. There is a valve the regulates the amount of coolant going into the heater exchange box, if you have the A/C on and temperature set to the coldest, then this valve should be closed and not allowing any hot coolant into this exchange, where if your heater is set to high, it should be allowing hot coolant into the exchange box.
I want to say there is a blockage somewhere in the coolant system, I guess you could check by draining the coolant, checking the amount that drained out and comparing it with the amount that the system is supposed to hold.
That's about all I can think off at this moment (as I'm quite tired- you can probably tell since my post doesn't sound very well structured), I'll have to think it over.
Cheers,
Matt.