I guess it all depends if you want to shoot for "looks great" or merely "playable". The absolute "minimum" system requirements include 512MB of RAM, a 64MB DX9 capable video card, and a 1GHz processor. At bare minimum this is what you will get you running, poorly. If you are looking for a good experience, with decent performance though I would recommend at least 1GB of RAM (2 would be even better if you have the budget for it) and a 256MB video card. This can get spendy if you aren't careful, but is well worth it.
As for Vista, EQ2 does work on Vista, but there have been some reported problems (mostly permission-based errors). Be sure to install and run everything as the Administrator account or with admin permissions and you should be good to go. If you run into any issues there are some lengthy discussions about it over on the official boards with some solutions.