I'm assuming that you're picking one of the columns of stats and that you're not taking into account your stat boost for 4th level. If this assumption holds true, I'd probably opt for the middle column.
5th level leaves things kind of open too. Too low for (most) prestige classes, but close enough that you want to plan now if you're going to enter one.
Caster is kind of general too. But I'll throw some ideas out:
1. Conjuration specialist. Go all out for summoning stuff. Take the option from Unearthed Arcana to speed up your summoning times, Conjurer, focused specialist (from Complete Mage), Augment Summoning feat, and there is a feat from Frostburn that makes your critters cold for +1d6 damage too. A twist on this is to multiclass Conjurer/Cleric, take a couple of domains that complement the theme (there are some good ones in the Spell Compendium, just can't think of them while at work), and go mystic theurge at level 7. Lower level spells yes, but you'll have tons of spells to summon stuff. There are tons of uses for summoned minions.
2. Archivist. Sometimes overlooked from Heroes of Horror, it has some nice abilities and potentially a very flexible list of spells.
3. Cleric/wizard and aim for True Necromancer (Libris Mortis). Lots of good things to do here.
4. If you don't mind dwarves, what about the Runesmith (Race of Stone)? How does fully armored spellcaster with nio arcane spell failure chance for a 1 level dip sound?
5. Cleric of a war deity. Sometimes overlooked, clerics following war deities can be quite nasty in combat, especially if they aren't wasting their spells on healing and picking ones to aid themselves in combat instead.