Are you still using Windows XP and want to upgrade?

Here are the official Vista requirements for all versions except Home Basic..

1 GHz 32-bit (x86) or 64-bit (x64) processor
1 GB of system memory
40 GB hard drive with at least 15 GB of available space
Support for DirectX 9 graphics with:
WDDM Driver
128 MB of graphics memory (minimum)
Pixel Shader 2.0 in hardware
32 bits per pixel
DVD-ROM drive
Audio Output Internet access (fees may apply)

Those are Microsoft’s recommendations. If you plan to play games or run a lot of memory eating applications my personal recommendation is to get at least 2GB of RAM and a 2Ghz+ CPU. If you want you can run Vista with just 256MB, but Vista is a real memory hog and you will not enjoy that slideshow.