If you plan on having Sand and Qara in your party at the same time, try to make them compliment each other. Qara has the ability to cast a few spells many times. It sounds like you have been focusing on spells which deal out a lot of damage. This is a common and useful way to employ a sorceror.
When you pick up Sand, fill up his spellbook by scribing scrolls you have found along the way, or perhaps buy some scrolls from local merchants. (Ironically, you could actually be buying srolls for sand from his very own shop!). When you choose spells for sand to learn, look at some defensive spells.
Some good defensive spells are:
mirror image
stoneskin
energy resistance (especially if you are going up against other casters)
mage armor & improved mage armor
There are more at higher levels and you should look at them. A well prepared wizard is almost untouchable.
You should also consider types of spells which disable opponents. If you are traveling with characters who are good in a melee fight, like Khelgar, if you can stun or hold an opponent for a few rounds, the effects can be devastating.
Some good disabling spells:
sleep
cloud of bewilderment
web
entangle
mass slumber
stinking cloud
everard's black tentacles
hold person
hold monster
Out of this list, I choose Cloud of Bewilderment, Stinking Cloud, and Everard's Black tentacles. These spells will incapacitate entire rooms of opponents, especially if you cast more than one.
Also consider domination/summoning spells.
These spells add an extra body to the party and can be helpful.
The best summoning spells are Summon Creature 3,4 and the one that summons a dire bear (7, I think). Gate is also excellent (level 9), but you probably are not there yet.
At fifth level, you get charm person which is huge. If you are going against a creature with low will save (any fighter type, and most monsters) then you can dominate them and add them to your party for a time. This works very well against powerful fighters.
For higher level spells,
The Bigby's hand spells are very effective against single powerful opponents. They are not so good against lesser opponents since they only affect on at a time.
Finally, when you hit 9th level, if you have purchased Storm of Zehir (or maybe it is Mask of the Betrayer), you will have access to a spell called Shadow Simalcrum. There is no save for this spell and it works on anything that is not more than 2x the level of the caster (so basically anything is suceptible). It makes a copy of the creature you target which will fight for you. The copy starts with 3/4 hit points, but you can heal it back to full hit points. I find this spell very effective against very powerful opponents.
In short, Wizards are useful for their variety of spells.
Good luck!