The windows Vista Min. requirement:
http://www.microsoft.com/windowsvista/getready/capable.mspx

Dont forget to check the Graphic card min. requirement too!

2GB is more than enough to Windows XP, i think 2GB or 4GB Ram , u really can see the different when operate your windows, windows also wont consume all the ram you install...if really want a good performance PC, why dont looking for Pentium D or P4-EE? I think you will feel the different experience!!!