I know first hand that Cod4,Cod5,NFS undercover,Crysis/warhead,Grid,Gta 4,CSS,Pure,Dirt,and Fallout3 all work just fine.
The only game problem I have ever had with 64bit was on Winxp 64bit with Guitar Hero III as they made it so that the installer wouldn't let you install it because the OS was not supported. This was simply taken care of by editing the MSI file I think it was to allow it to install and then it worked perfectly.
On vista 64bit this did not happen for me and everything worked just fine,I have not had any application or game on vista 64 give me a problem yet and have been running it for a few months now.
As long as your not using some old outdated piece of hardware in your machine that will require a 64bit driver then you will be fine with the switch. For 64bit drivers for vista and a lot of applications that are 64 bit you can get some from
http://www.start64.com/index.php
I suggest getting all your drivers right before you format your machine and put them on a different partition or on something else. All applications that are not 64 bit will be installed in a folder called Program files\(x86) and the 64bit versions go to just normal /program files.
As you install them the installer will put them where they belong but it really doesn't matter,they should all run without problems even if they are 32 bit. I have about 30 applications installed on my machine that I use and only 4 of them are actually 64bit not including drivers.
Just make sure you are using at least 4 gigs of ram and you will be all set,games should run just as well if not better especially with being able to use all 4 gigs.
