Couldn't resist to post even later than "Rob the late post"
I switched to Vista 64 bit as soon as it was possible because that allowed my PC to facilitate more than 4 GB RAM. Due to adressing limitations of 32-bit operating systems (2 to the power of 32 = 4 GB) the graphic adress space plus the OS adress space reduces the available adress space for programs (which is one reason why your XP system info tells you to have less than 4 GB available RAM even if you do have 4 GB of RAM chips plugged in). With graphic cards RAM getting bigger and bigger all the time, 32-bit OS are no good investment into the future (and XP 64-bit was never stable).
From what I've heard Windows 7 is indeed improved over Vista, but I'm not only lacking the money to upgrade, I flinch from re-installing all the programs. My PC runs fine and fast, so I don't have the urge to change an already stable system.
Back to topic: I am probably the only one not using a single mod yet. I have Sims 3 on autoupdate and never had a crash or anything. My fear is that even if proven mods have never caused any problem, I am sure the moment I'd picked one the next update would cause it to crash my game