Man u will need to spend atleast 5k on the graphics card alone to be able to play Doom3 decently. U could go for a 9600 which is available for that price. Then u would have 11k left. Now, u need atleast 512MB RAM. That might cost, say Rs. 3000 (karnataka prices, will be cheaper elsewhere) SO then, u will have Rs. 8k left. WIth that u need to get a decent cpu & motherboard ( I am assuming u have all the other things like HDD, CDROM etc & they don't need upgrading). Get a cheap motherboard like Krypton C18G - 400 ( available for around Rs. 3000/- ) The remaining 5k can go for CPU. Go for Athlon XP 2600+ or something like that. I think that will cost Rs. 5k. Please check local retailers for pricing info.
Thats it, u are done. It ain't the best of specs, but it WILL fulfill ur requirement of being able to play Doom 3 at decent settings (800*600 resolution, high setting) in ur stated budget of Rs. 16k. As others have stated, it is always better to spend more & go for Athlon 64.