A new average computer for a dedicated trading machine shouldn't be challenged by charting/trading at all if you use XP. Exceptions are heavy system development and backtesting, computer intensive indicators e.g. multi timeframe, multi instrument and automated trading if your strategy is very complex ( or just just poorly implemented
, applies also to all others ).
I think MS is creating an artificial demand for their friends in the hardware business for the none gaming people by forcing Vista on them.