can any one please tell me if Nvidia GeForce 9600GS will be good enough to play WAR & Warcraft (Lich King) smoothly without lag at decent settings, will be paired with a 24 inch dell * Full HD 1920 x 1080 resolution
* 16:9 dynamic aspect ratio
* HDMI high quality digital connection
You've got a bit of a problem there. That sort of PCs usually come with small PSUs and small cases, both of which could prevent you from upgrading to a serious gaming card. Do you have the PC already? Try to see how much room you have for the video card. You need about 9.5" for a HD 4870 and about 11" for a GTX 260, I think. Also check the PSU parameters - they should be written on the back of the PSU. You need something like 500W or more for HD 4870, and 525W/(38A on 12V) for GTX 260.
If you have to go for NVIDIA (It does not offer SM 4.1 & DX 10.1) Go for ATI & you'll have the latest SM & DX. Anyway, if its NVIDIA, 9800GTX, ATI - HD 4850
Your card's main problem is its memory bandwidth, it runs DDR2
But IMO you can stick with it if its just Warcraft that you're gonna play