I'm in the same boat.  not an expert, but here's what is found so far:

   (http://www.rme-audio.de/forum/viewtopic.php?id=21594)

1) if you're retiring the PCI based computer you will need a new PCIe version of the card (typical price $549 - so thats the minimal price you're looking at).

2) There are drivers to upgrade multiface to Win 7/64. 
    The new PCIe should  then work with the old Multiface, but you must make sure your firmware is up to date.  If the hardware version is very old, there is at least a slight chance it may not work; and the card would have to be sent in to reprogrammed.  You can find the details on their site under the older drivers section.

That scared me enough to think about upgrading to the Multiface II.  After a dozen years i can't complain.  plus then I can keep the old computer intact with the old interface too.  But it appears to me that Multiface II is now also pretty old and  getting phased out.  ( some major distributors don't even carry it any more, but i saw one at a substantial discount)

RME tends to be very vague on performance differences, but it appears they feel that their USB2/ firewire solution is no longer inferior to the old proprietary system of the PCI-MF . ("it all depends on your system") 

With that said, the Fireface UCX / 802 / UFX seem to be a more future proof direction, but I'm still figuring this stuff out.   
(The UCX resembles the MF's, typical price is around $1600)

OK then its just part 2 you need to consider. 
My new win7/64 PC has a 1 PCI slot, but I would need to deal with the ominous firmware upgrade warning.
" Note for HDSP PCI Rev. < 1.8  ...  "

Pop the card and see what's printed on it.
Mine was 1.4   If yours is > 1.8 then I'd breathe easier.

I installed it under WinXP x32 and ran the firmware updater.
Win7x64 drivers? Anyone?
C