No it does not mean you'd have to buy a new processor. The Athlon 64's can work in both 32bit and 64bit, so the old windows will be fine with it. Don't worry
Not many programs are 64bit yet, and when this finally happens and people decide to get the 64bit windows version when its worth it, you'll get a boost with this processor, and unlike other people without a 64bit. You won't have to update! Great huh?