@ Zazabar
Mixing different sticks of ram is not a good idea. You can expect freezes and BSODs.

@ rumeshkumar
Start a new thread!

New ram, as G.Skill?

Also, your CPU might be the root of the problems aswell, but nevertheless, if it isn't RAM, then my money is on mobo.

CPUs have implemented memory controllers. As far as I understand, you problems come to light when you plug in 2 Gig sticks. Memory controller is one responible of allocating memory addresses (read: using those gigs) and that is where the faliure might occur in your case, but I kind of doubt that it would be the case with 2 Gig sticks only. It would likely show with any sticks.

My advice is:

If the mobo warrany is still valid, take the PC to the same PC shop you bought your mobo, or (if bought online) to any PC shop that is willing to RMA your mobo (since it is probbably mobo). Staff there will quickly diagnose the problem, and they will replace it and bring you the bill...

If the mobo warrany expired, expect a bigger bill.

No. It would be buying a new mobo. Only difference between that and actually buying a mobo is that i+your PC would get positive confirmation that it is a mobo that needs replacing.