Yes, it is that simple. But installing a new OS would require you to format your hard drive and reinstall ALL of your applications and settings (see FAQ). And if you are upgrading OSes you might as well bump up to Windows 7 x64 since it is much faster and more stable than XP x64.
I agree - don't go for XP x64
I have XP x64 on the computer, that runs SW - I have lots of crashes and strange behaviour (fx. all closed SW files from the current session use my RAM - up to 6.5GB!! and no, the files aren't that big) and my VAR tells me, it is because Win XP x64 is unstable and hasn't been supported by Microsoft for more than a year. I might be able to use SW for more complicated calculations with 8GB RAM, but my SW is slower than my colleague's on an old machine with 2GB RAM running regular XP.
Besides SW2011 won't support XP, so I would definitely go for Windows 7 right away.
It depends on what kind of computer you have, if it's compatible with more RAM, 6Gb, or even better 8Gb. Then you can install the new RAM and boot the computer with a installation-dvd containing 64-bit windows (xp, vista or win7 is availiable in 64 bit) I would recommend win7 or xp...
It's really not that complicated at all, I've done it myself on 2 computers...
The first thing you need to do is check the spec's for your computer, the hardware must support x64 software, this can be confirmed by the manufacturer. This is critical, you cannot take a 5 year old P4 computer and just put x64 on it.
If your hardware is x64 compatible, then go for it and as others have said, go with Windows 7 if you can. Again, you need to look into a few things first. Windows 7 is new and there are not drivers for all components (printers, scanners, video cards...). Plus some of your other software may not run on Windows 7, look at each application you have installed and confirm it will run in x64 and on Windows 7 before reformatting anything.