Malicene said:
I think crit damage does not increase spell damage as stated before.
I don't think it's nearly as clear as all that. Ordinarily, you can crit with any spell that requires an attack roll and inflicts damage. Ordinarily, a spellcaster can also deliver a touch range spell with an unarmed attack (provoking AoOs unless he has Improved Unarmed Strike). That requires the caster to attack the normal AC of the target rather than the touch AC and enables him or her to add unarmed attack damage to the roll. I think it's pretty clear that a crit with such an attack would multiply the damage of the spell as well. The spell damage is still spell damage rather than additional damage dice like that of a flaming weapon.
The spellsword channel ability explicitly states that you are casting the spell as a free action and delivering the spell through the weapon. Since the wording is the same as delivering a touch spell with an unarmed strike, it seems pretty clear to me that the damage should be multiplied on a critical hit. The big question would then be whether the spell benefits from the improved crit range of the weapon used to deliver it (I'm pretty certain it wouldn't benefit from any improved crit multiplier).
spell to channel :
any damaging spells (burning hand/grasp/missile...)
Web/blindness
Flaming sphere
Gazeous Form ! (think about it...)
Melf acid arrow : beware with Ranged/contact touch attack : channelling it you lose the "touch attack bonus" (no armor)
Stinking cloud
..... to be continued
This list is far too long and includes a lot of spells that actually can't be channeled. A channelled spell must have a target of creature or creatures. Consequently area effects like Burning Hands, Web, Stinking Cloud, etc are out. Flaming sphere is out for a similar reason.
As far as Acid Arrow goes, it's a rather poor choice for a channeled spell. Before level 6, it deals 4d4 damage over two rounds--only slightly more than Magic Missile's 2d4+2 and less than 3d4+3. In addition, you could channel two magic missile spells instead of the single acid arrow.
Really, the lesser orb spells are probably the best alternatives to magic missile for channelled spells. They yield slightly more damage on a failed save and slightly less on a successful save. Ray of Enfeeblement or Reduce would also be decent choices if you have a target likely to fail a fort save. If you were in a cramped space, enlarge might also be a good choice--it doesn't grant much strength and you might be able to convince your DM to give an enlarged target significant penalties for fighting in a small area.
