Magestrike is pretty cool... it's not the way I would go.
Actually, the way I *went* when I thought this was to simply give mages increased access to cantrips, I think that I gave them bonus cantrips for intelligence, and then multiplied the number they could cast per day by 1/2 their mage level. Which worked pretty well. You never saw a mage using a non magical attack again. This can be really important as those ranged touch attacks make a real difference in higher levels. And I don't think it's too powerful to give it to 'em.
On the other hand, to go this route, and just give them a magestrike type ability. Personally I would stat it differently.
Have it do d3 damage, and be a ranged touch attack, just like the relevant cantrips. Give it all the stats of a ranged touch attack cantrip... but have it be physical only, and for that matter, give them INT mod bonus to damage...
Hrmmm...
OK, or how about this.
Magestrike... you can manifest magical energy as physical force. Damage d3, bludgeoning (physical), DOES count as magical for DR (hey, the mage should get a bonus for that! It's clearly magic. Sure low level mages *should* be good for hitting the ghost... once. Then dying a horrible death as the mage remembers that pitiful hit die) If manifested as a melee weapon it also gets int mod bonus to hit and to damage, if manifested as a ranged attack, it has the range increments of a crossbow, provokes AoO's, recieves + int mod to damage, and it can be manifested as a ranged attack a number of times per round equal to the number of ranged attackes you would normally be able to make as determined by your BAB.
This makes it useful both as a melee and a ranged attack, does not give it touch attack bonuses, but does give it bonus to hit similiar to a composite short bow... and gives is a longer range than perhaps it should have... OK, make the range appropriate for a cantrip: Close.