Let's see...
Drow Mage -- assumes fractional BAB, else we can't get into Spellsword; assumes two flaws for two extra feats at 1st level
1/ Ranger -- Combat Casting, Iron Will, Dodge
2/ Wizard (Diviner) -- [Scribe Scroll]
3/ Drow Paragon (SRD) -- Practiced Spellcaster (Wizard)
4/ Fighter -- [Toughness]
5/ Rogue
6/ Unseen Seer (C:Mage) -- Spell Focus (Divination)
7/ Master Specialist (Divination) (C:Mage) -- [Skill Focus (Spellcraft)]
8/ Mindbender (C:Arc)
9/ Candle Caster (T&B) -- Mindsight
10/ Spellsword (C:War)
11/ Dragonslayer (Draco)
12/ Abjurant Champion (C:Mage) -- Spell Focus (Abjuration)
13/ Arcane Trickster (SRD)
14/ Geometer (C:Arc)
15/ Fatespinner (C:Arc) -- Greater Spell Focus (Abjuration)
16/ Initiate of the Sevenfold Veil (C:Arc)
17/ Archmage (SRD)
18/ Wayfarer Guide (C:Arc) -- Great Fortitude
19/ Blood Magus (C:Arc)
20/ ... ? (I'm at a loss here)
Assuming we can find another PrC which advances spellcasting, we'll get 9th level spells at 20th level.
Cheers, -- N