actually, sony phones arent exactly smartphones.. and if u have been using a symbian nokia phone, it would be hard to move to SOny UI...
K800i is just launched, but as it is not a walkman series phone, i think W810i would have better sound quality. The camera on the W810i is the same as the K750i, so its quite good...
Audio quality on N70 is quite good, N72 no idea, but it is more or same equal to N70 so there shouldn't be a price difference..
Lastly... N73 is definatly a phone worth waiting for if, but it has got an equal in SE K800i .... and N73 is suppossed to be launched in the coming week, so take it a month before it gets here and the expected price for it is 30k +
I would suggest u check out N80 also... its one hell of a phone.. only downer its cam hasnt got Autofocus but fixed focus, which however workd surpisingly well...