A Gish serves a pretty specific role in a party, and 11/12 times, its not the primary arcane caster. A Gish can make an outstanding tank using spells to suppliment AC and HP. A Gish can make a good damage dealer, using spells such as Wraithstrike and/or Whirling Blade and/or Arcane Strike in conjunction with Power Attack and offensive buffs, then he serves a role similar to Arcane Trickster builds. A Gish can even suppliment support, tossing up spells like Mass Fly, Evards, and Solid Fog that don't have saves. That said, a Gish is seldom batman, and almost always breaks the first rule of optimization (thou shalt not lose caster levels!!!!). The biggest weighting factors in a Gish is trying to balance caster levels (more and better spells) with BAB (for Power Attack damage). Some Gish builds value one more than the other.

As far as Gish builds, I've heard quite a few, from the simple:
Fighter/Wizard/Spellsword/Eldritch Knight
To the complicated and sometimes obscene:
Paladin/Sorc/Swiftblade/Eldritch Knight
Paladin/Bard/Sublime Chord/IotSV
Paladin/Bard/Dragon Disciple

The most important things for a Gish is to pick a couple tricks and stick to them. You CAN'T get 9th level spells with DC40 saves and 20/20 BAB. Its just not possible. You CAN get enough of both to be VERY effective, with a little careful planning.