lvl 21 is 975,000 GP.
When a Wizard goes from lvl 20 to lvl 21, they get:
1 Epic Feat (Instead of a Normal Feat)
2 new spells known
1 Hit Die.
+1 Attack bonus (Not Base Attack, just a +1 to attack)
Same skill points as other lvls.
Your Familiar's Natural armour bonus becomes +11, theirn Int becomes 16, and they gain the "Familiar Spell" ability (See description below)
If you don't have the Epic Handbook, then for the Feat I'ld suggest taking 'Improved Spell capacity (Allows you 10th lvl spell slots in which to memorize spells, such as Heightened Hold person, Quickened Disintegrate, Maximized Delayed Blast Fireball, etc, etc.) You'ld get 1 tenth level slot, + more depending on what your Int bonus is. Every time u take the feat, you can cast spells of one level higher (Twice you get lvl 11, three times u get lvl 12, etc, etc).
If that's not your bag, then you might like Familiar Spell (It's an epic feat that does the same as the Ability of the Familiar, letting you pick a spell you know of 8th lvl or lower. Your Familiar can cast that spell once/day as a spell-like ability.
Also, if you like Quickened Spells, then MultiSpell works nicely (Each time you take it it lets you cast one more quickened spell per round)