These guys realy benefit from the Marine Horde principle-basicly max out as many troops as you can (Thousand sons dont take heavy weapons so It fits them perfectly) and with two wounds a piece (at the moment) they can give your opponent a real headache!
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Marine Horde Principle
For 1500pts a Thousands sons horde could have
60 marines with 2 wounds-quite a nightmare!
It might be worth waiting till the new Chaos codex comes out before doing any major army modifications or going on a spending spree, theres new rules and new goodies coming out so....rumours of 4+ invulnerable saves,hellfire bolts etc!