Here are my current 4 years old specs
radeon 4870 2gb
Can i keep the current specs and change only the GPU? will i be able to play current games smoothly 1080p with just a new GPU? or there will be bottleneck?
can you recommend a good GPU budget $200 - 250
Sorry mate, this will most definitely bottleneck.
Best thing to do would be to build a new system, if you post a budget I could suggest something.