After my debacle a couple of days ago removing my RAID array, I was thinking about how undramatic the shift to 64 bit has been for me.
I moved my server and principle development PC to 64 bit over the last few weeks. For the server I bought a completely new machine and installed 64 bit from scratch. My server and desktop are similar specs, both having quad core processors with 4Gb RAM (I'm thinking about upgrading the RAM further...)
In general, the move to 64 bit has been pretty simple. Almost everything I have installed works without problems. My desktop PC feels faster now and certainly gets to the Windows login screen much faster than it used to. It is tricky to specifically attribute that to the 64 bit move as I completely reinstalled Windows which always makes things move a bit faster! However, weeks (and several program installations) later, things are still moving along pretty quickly. I even installed Elder Scrolls Morrowind which runs without a hitch on a 64 bit platform!
My only real problem as been IIS 7 on a 64 bit platform. On the server I struggled to get Outlook Web Access working (and any other web site). I had to do a bit of Googling, but I eventually found out how to configure the server to permit 32 and 64 web sites to run side by side.
So overall, I would recommend looking at 64 computing for developers (and Morrowind fans).