Don't know much, but here it is.
I think you should talk about him in relation to the 2nd invasion of greece, namely his failure to take over greece and his distastrous losses at the battles plataea(479) and Salamis(480). Part of his 'legacy' is not only his failure to win this war, but also the scale on which the Persians were beaten and the consequences of this. Salamis (a battle which he viewed) was an important hist. turning point, as it shattered the Persians' naval hegemony of the med. sea. It also represets the start of athenain naval hegemony, and is a key contributer to the genesis of Greek dominance and prosperity which lasts till the peloponnesian war. As Xerxes ordered this battle and the war as a whole, the defeat and failure of the Persians (and subsequent consequences) can be viewed as "his" error, his fault etc. This is part of his legacy.