new graphics card

Hi Im looking towards upgrading the graphics card on my HP pavilion 762n (new computer is out of the question). I need some advice on which card to get, 126 mb would be nice. I currently have a nvidia geforce 4 mx420 (AGP). Also do i need to change the power supply on my computer? I think my pavillion has a 200 watt supply. Should I upgrade the psu and card by myself or send it to a shop? Thanks.