ewans - first question, why replace? Are they too far gone, can't they be tidied-up?
To remove (are you sure they're T&G and not plain edge). First question is any hammering going to damage plaster ceilings below? A cheap hardpoint saw from a DIY shed, a couple of levers (thin crowbars), a bolster chisel, club hammer - these are what you'll need to take it up. Hire a nail puller to get all the orphan nails out that will remain in the joists (there could be loads of these); you could use a claw hammer but our experience shows a puller is more efficient. Brush the dust off the joists. Safety, take the time to remove the nails in the old boards as you are bound to stand on one (been there and done it - penicillin jab in the arse down at casualty also). If space is tight lift half the floor and replace.
To replace. Remember the hammering question above! Choice - traditional nailing through tops of boards or secret nailing through tongues. Trad nailing, hammer & nail set (punch). Secret nailing - hire air nailer, compressor (don't forget the special nails to go with the nailer). Whilst you're down at the hire shop pick-up a flooring clamp, used if you need to nudge a board or two, although you can get away with jamming an old chisel into the joist edge and use it as a lever against the board.
