i'm currently using an ati 4870 card in my system which as got 500watts smps...................now since i newly bought another ati 4870 card which i'm going to use in crossfire in the same system i'll need a total of 600watts smps which would cost me Rs.4000 extra....................so i was wondering if i could use another 350watts smps which costs me only Rs.1500 for the new card along with the existing 500watts smps for the older card and motherboard in the same system.
It's not impossible.
But it's not a good idea to use a 2nd power supply in a normal PC system. They're just not designed for that type of thing.
You'd be better off using a dedicated graphics power supply that you can mount in a your cabinet like a DVD drive.
Once example is FSP Booster X3
But you might find it less expensive just to buy a replacement power supply.