I bought ATI Radeon HD 5970 for More Graphics memory of 2GB. But iam not able to effectively utilize the 2GB RAM. This Card is having two GPU's with 1GB Each. Is there any way to utilize the 2GB RAM at a stretch
If both cards have 1gig each you only really have 1gig of ram period. The 2gig thing is something graphics card manufacturers seem to be able to keep getting away ( marketing ploy ) with when in actuality there's still only one gig available. If you want 2gigs avail you'll have to buy a 4gig card. Same thing with cross fire or SLI. Two cards don't mean twice the memory to use.
Just to explain it a bit. What happens is you buy a card like a 5970 which has 2 GPU's on board. This is to all intents and purposes the same situation as far as memory goes as running 2 cards in Crossfire.
Each GPU or card has to have a copy of the same information in it as the other one, so they both know where they are up to.
This means that if you have a 1GB card and add another one you still only actually have 1GB of ram available as by the time the memory on card 1 is full card 2 has an exact copy as well.