I've seen some powerful wizards played in the campaigns I DM. In each case, it's really not due to skills, feats or spells, but much more due to their stats and race.
First of all, Elves make terrible choices for wizards. Why? The -2 con penalty cripples them too much. So although their racial favored class is Wizard, this just isn't the optimal race to use for a wizard. So what is?
Dwarves make really good wizards. From fantasy roots, there's not much precedence for this, but with the 3.5 rules, they are simply one of the best races available. Why? +2 con bonus is invaluable. On top of that, they get the +2 saves against spells and poisons (useful for someone likely to be targetted at range by the enemy) and darkvision lets them be useful underground w/o light sources. Also, they are medium size, letting them use regular sized weapons. Besides having a high intelligence, pump the constitution and then finally dex. With a high con (say 16 or so), the dwarf wizard can typically have as many or more hit points than clerics or rogues in the party. Meanwhile, the -2 penalty on charisma is minor as its largely a dump stat for most wizards.
Theoretically, gnomes should also make decent wizards, but for some reason they aren't as good. Though the do get the +2 con bonus, their racial modifiers are nowhere near as good as the dwarfs. Low light vision is handy, but it almost always less useful than darkvision. And being small-size means they have to use smaller-sized weapons. The -2 penalty on strength is slightly more noticeable than the -2 charisma for dwarves.
While int-enhancing gear (Headbands of Intellect, etc) is highly desireable, the wizard should make it a priority to find Con-boosting items (Amulet of Health, etc). Armor class is also important, but you shouldn't be getting involved in melee, attacks, and any ranged attackers should be dealt with swiftly. If you have a choice between an AC-booster and a Con-booster, I'd take the Con item first.
Wizards or sorcerers with low con values may survive for a while, but eventually area effect and other damage spells or attacks will catch up with them. It doesn't matter how cool your feats, skills and spell list is if you're not alive to use them.