I used 'Windows6.1-KB958559-x64.msu' and then followed up with 'WindowsXPMode_en-us.exe'. Installed just fine on W7 RTM 64-bit. To be honest I find Virtualbox in Seamless mode a lot quicker and more responsive than XP mode.
Is it runs seamlessly(well sort of). You don't have to start the virtual machine first as you just click on the shortcut or menu and the program executes. The XP mode is great for compatability of programs and a virtual machine is great for other OSes.