evgeny, US$5900??? In case it helps, if you look around, you can also get the Imacon branded or Hasselblad branded Ixpress digital backs - the 96C and 132C backs for between US$6000 - US$8900. Just that to me, if you're spending as much as 6K, you start to have options... Just like the Leaf Valeo, you get to lug about a hard disk pack.
These 2 will mount on your Contax 645:
96C: square Kodak sensor, 16MP, i-Adapters to mount on lot's of different systems, upgradeable to multishot.
132C: 22mp Kodak 48x36 sensor, i-Adapters, upgradeable to multi-shot.
The firmware is still getting updated in recent FlexColor updates.
FYI, if you see a V96C, it is similar to the 96C except that it will only mount on a Hasselblad V body and does not have a multishot upgrade option, but it tends to be cheaper, around US$5200.
Jason