You may want to wait foro Windows 8 unless you want to buy Windows 7 now and then go to 8 later. Windows 7 will cost you $100, but Windows 8 is initially going to only cost $39, I believe.
For one thing, if you're still running XP, you cannot upgrade to Windows 7 or 8 in place. That means you need to backup, reformat your disk, install Windows 8, and then reinstall all of your applications and then restore the data from the backup.
What I did when I upgraded to Windows 7 was to buy a new disk drive and installed Windows 7 on it and then installed all my apps and copied the data from the old drive. That way if I had problems with Windows 7, I could still boot from my XP drive. I didn't have any problems and so eventually I reformatted the old XP drive and used it as an additional drive.