Another thought on the Cubase vs Nuendo question - apart from the extra sync/import/export options, and the amazing new automation, the other trick Nuendo does is Warp to Picture - similar to the Audiowarp in Cubase, but for frame-accurate tweaking - an absolute godsend when the director comes back with an edited scene that is a second or so shorter, as you can just click and drag your music back into line! Wonderful!
Check the listing on Steinbergs site for the things that do and don't work in 64-bit - I agree with DG that it seems still a little too early to risk it.
As an alternative to the Lynx, the RME HDSP range of PCIe cards are fantastic, and the 64-bit drivers have been around for a long time, both XP x64 and Vista x64