Basically, your system need to be "The Bomb" if you plan on upgrading to Windows Vista. Here are the basic you should have to run this new OS. Keep in mind that this seem overkill for some of you and may or may not change:
-2GB RAM
-64Bit CPU (I think it's safe to say we all have that)
-PCI-express graphics card with minimum 128MB of onboard RAM
-SATA with Native Command Queing
-Monitor that supports high definition playback
Seems a lot I know. Also, Microsoft plan on releasing 7 flavors of Vista ranging from an "ultra stripped down version" for third world countrys to a full featured "ultimate" edition with all business and multimedia components included.