Dont downgrade esp. for thos games, HL2 and halo2 will defenatly run with the eye candy you'd expect....with a 6800gt , that is if you have sufficient ram and cpu speed. the 6800GT is unmatched anywhere! you will be kicking youself so bad when newer games come out and you need that extra juice your 6600gt cant provide but your 6800gt gives.
Although if your very tight on budget you could go 6600gt pc-i, but i dont know the performance of those but i expect it would be a fair drop from 6800gt.