You wont make it anyway for a full onyx set, being at 1800 bucks for the 1640 including firewire ( 1620 is immediatly lesser condidate for a convenient multitrack setup). Considering that's only 96K is expensive since it's "outdated" but on the otherhand you're right, there hardly any difference and most studios even record simply at 44.1kHz/24bit (less clocking issues, next step would be 88.2kHz and so on which is preferable).
Still, it's the only kit around except for a SSL 900AVS that has converters embedded into the analogue mixer with digital connection. An alternative means a seperate console alike the onyx which is the 1604VLZ ( but scope out A&H Mixwizard 16:2, much better eq IMO, more connectivity ) and any means to record 16 I/O's, preferably up to date, at 192kHz. The sum of that would exceed 1200, hell even 1800 depending on what you're going for.
Next question; you got a lot of digital output available instead of analog ? MPC4k and other gear/fx ? and which has the highest resolution ( I assume it's either 48/96K ) ? If you figure out how much digital connectivity you require the setup could be changed to your budget, if not for the better ( hey, it's fibre optics, no static, no noise! ).