Technically, having 1 PP in the weapon you use to backstab doesn't reduce the damage that much over 2 PP. 2 PP gives you +1 attack and +2 damage, which isn't that much considering backstab gives +4 attack and only multiplies base damage, not the extra damage for specialization, strength, or magical weapons. I've had my F/Ts use weapons they were only proficient with to backstab if the said weapons were worth it (poison dagger, mainly).
However, if you were to have 3 or 4 PP in a weapon, I'd definitely try to have them in a backstab-capable weapon - that extra oomph will increase your chances for a hit AND a clean kill. That said, as you seem very attached to your greatsword,
go ahead and just use those thief PPs for backstab weapons. It won't be a problem with a Lvl 9 fighter's Thac0 and exceptional Str added on to the backstab.
You will need a set of leather or studded leather. If in chainmail or above (as a F/T with all skills regained), all the thief skill buttons are greyed out, with the exception of detect traps (which presumably is based on observation, not dexterity or silence). One thing you could do, although it's slightly cheap, is to carry two sets of armour; one set of leathers and one set of plate for when your F/T enters melee. You can even pause the game and change armour right after you backstab. Alternately, just wear the boots of speed, and you can outrun the opposition instead of fighting it out after you backstab.
You could also use Sneak Attack instead of backstab - it does less damage, but is more generally useful. You get bonus damage for hitting enemies on their flank or from behind, although you don't get the backstab damage multiplier (even if it is a backstab situation).
[ 11-21-2004, 05:11 AM: Message edited by: Aerich ]