What are the characteristics of a Vista capable computer?
Technically, any computer that meets the minimum requirements is "Vista-capable". In reality, you may find the listed requirements too conservative to run Vista at an acceptable level.
Intel Pentium III or compatible @ 800 MHz
512 MB RAM
20 GB hard drive (Vista needs 15 for itself)
DirectX 9 compatible video card (actually, any video card that supports VESA will work)