Coupling of two Graphics Cards Possible?

My friend has a Gaphics card of model 'Nvidia Geforce 8400 GS'. He needs to upgrade the graphics card as the current one is not enough for some games. So can we couple two Graphics cards like we couple RAM (eg., 2 x 4GB RAM). If yes can we use different graphics card or should we use two similar models?. Plz give your valuable suggestions.

You can use up to four GPU's working together. It will be SLI for Nvidia cards and Crosfire for AMD cards. To do that you'll need same GPU models and proper mainboard. An of course good supplier.
In my opinion I can't see any sense in this if someone don't want to create a real monster PC and use Eyefinity etc. I think it's better to invest in one stronger card than two weak and 8400GS is really slow card. Investing in one strong card which will handle with all new games you'll pay less and use less energy.

Yes. The coupling of two graphics cards for AMD is called Crossfire and for NVIDIA is called SLI.

Unfortunately, this doesn't help your friend because the cards that are 'coupled' need to be of the same series.

The 8400GS is so old and crap it makes no sense to get another one and couple them. In fact, i doubt you'd be able to source a 2nd one.

Also, the motherboard and the power supply must support it.

brothers what about this as i am using i7 processor with intel HD inbuild graphic which is 512 mb and external graphic card of nividia g force gt 540 m could this be suficient for good animation work and games

rashmigupt, from what I know it is not possible to use both the integrated GPU (Intel HD) and the external at the same time. You can use only similar external GPUs in SLI or Crosfire.