Well, there are a few basic keys to keep in mind.
Paladin - you probably want either level 2 or level 4, depending on whether or not you have access to Divine feats. Beyond that, sorta pointless.
Rogue - The skill points are handy, but it doesn't add much to casting or swinging a sword. I'd drop it for something that helps you with at least one... ranger? Swordsage? Heck, even a monk to get your wisdom bonus when fighting unarmored, which you'll want to if you can't get around Arcane Spell Failure. Plus, there's some feats for multiclassed monk/sorcerers and monk/paladins in Complete Adventurer. Keep in mind that without a few levels, you're not going to be able to do much as far as traps and the like go - the DCs can quickly climb out of your reach.
Sorcerer - If you're sticking to what you had in the computer game, you'll probably be focusing on this. So, as many levels of sorcerer as you need to get in to the prestige class(es) of your choice. Things like abjurant champion, with full BAB and full casting progression, should be your first choice.
A few things worth picking up:
Practised Spellcaster - make your spells more potent, make them last longer.
A Spellstoring weapon of your choice. You can put a targeted spell of your choice up to third level in here. While normally this means taking your best 3rd level targeted spell (or maybe a lower level one, like scorching ray) and slapping it in here... consider grabbing Sudden Metamagic feats like Sudden Empower or Sudden Maximize. Slap one of these on a third level targeted spell (probably have to hit up the spell compendium for something good) and you've got a weapon that can dish out one really, really nasty attack. Or, for something less traditional, toss Vampiric Touch in there - hit em hard and get bonus HP for it.
For spells: I was toying around the the idea of a paladin/sorcerer a while back who was going to use a fair amount of necromancy - false life and vampiric touch would have been his big spells at lower levels. Given, that can depend on your DM - talking about it with a few friends, two said they would not allow a Paladin to use Vampiric touch while two saw no issue with it. But it seemed like a neat idea.
Other spells to take... mage armor/greater mage armor (spell compendium) are obvious if you don't have a way around ASF. True Strike is sort of a 'duh'. Your basic gish stuff, really.
