The others mentionned it! 64bit support is a bonus that will give you a free upgrade to 64bit when you will be ready!
The real question is WHAT will you do with your PC? This will help you choose the right CPU you need! The apps. you will run on the PC will guide you in your buying decision and 32bit or 64bit will only be a "+" if you hesitate between 2 CPU or platform.
A7N8X / <font color=green><b>Sempron 2800+</b></font color=green> <- <i>Is this affecting my credibility?</i>
Kingston DDR333 2x256Megs
<font color=red>Radeon 8500 128Megs</font color=red> @ C:275/M:290 <- <i>It's enough for WoW!</i>