I believe the 8800GTX is not as good as the newer 9800GTX. I would also advise against SLI as it would probably be more profitable to just get a new GPU in maybe 12 months and have a better system. Unless, of course, the games you play now don't give you enough FPS to be able to play the games now, but if the 9800GTX is enough now, then I wouldn't SLI.
I do not know Hi-Power, so I can't comment on that, but most likely you don't need 1000 watt.
The Asus maximus is a X38 mobo which means it won't be able to SLI, but other than that, it is good.
As for the 8gb RAM, well it is probably excessive for now, but what exactly are you doing that requires that much RAM ?