Thanks for your reply. I have calculated the maximum wattage needed for my current config is 397 Watt. There is a link from MSI to calculate, http://www.msi.com/index.php?func=power.
If I want to add a Radeon HD4850 or Geforce 9800GTX+ then I have to buy a PSU of 500Watt. I will not go for Crossfire. As there is a tie in Tom's article with Geforce 9800GTX+ and Radeon HD4850. As ATI cards are weaker compared to nVIDIA when AA is enabled so which one should I go for? I am now planning to add a 500Watt PSU :-)