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 :-)
Get a good quality PSU; Corsair, PC Power and Cooling, Antec and Seasonic are amongst the best. Avoid the cheaper options, some do n`t deliver the power they claim and they often lack the protection of higher quality units.
Both 9800GTX+ and HD4850 are comparable in speed and price but because you already have an Nvidia card I`d say get the 9800GTX+, just because it`ll avoid potential driver problems during the install.
I`d suggest getting one from EVGA XFX or BFG (in order of preference) because they offer the best warranties and support, just make sure you register for the longer warranties they offer.
Yup, u r right. Currently the PSU I am using (Thermaltake litepower W0162) has power efficiency of more than 75% (as mentioned in the spec). I will buy a new one with effiency more than 80-85%. Thanks for your opinion. I am planning to go for Radeon HD4850 and a 750Watt PSU for future crossfire option, heh, heh.