Good piece of reference for the system requirements that you need in order to have Windows Vista running smoothly on your computer.
Windows Vista Capable PC Specification (Minimum Requirements):
A modern processor (at least 800MHz)
512 MB of system RAM memory
A graphics processor that is DirectX 9 capable
Windows Vista Premium Ready PC Specifications (Recommended Requirements):
1 GHz 32-bit (x86) or 64-bit (x64) processor
1 GB of system memory
A graphics processor that runs Windows Aero (DirectX 9 class graphics processor that supports WDDM Driver, Pixel Shader 2.0 in hardware and 32 bits per pixel with big enough graphics memory)
128 MB of graphics memory
40 GB of hard drive capacity with 15 GB free space
DVD-ROM Drive
Audio output capability
Internet access capability