I have a monitor with max resoulution 1680 x 1050? So just wanted to know at this resolution will there be any difference in performance of 4870 or 4870 x2 ?
That resolution? A 4870 would be more than good enough. You might want to check the titles of the games you play, because a GTX260 would also be a great choice (along with the 9800 GX2). To be honest, at 1680*1050 a 4850 is already more than good enough (plus it's $100 cheaper).
For 22 inch monitor, at 1680 X 1050, 1 X 4850 will perform great and is good value too (performance/$$). 4870 is great if you have money to burn. 4870X2 is too much $$ for very little gain.
Actually I came upon this benchmark here: http://www.pcgameshardware.com/aid,659033/Reviews/Stalker_Clear_Sky_DX10_Benchmark_Review/?page=2. In this the card's fps are shown, not good at even 1028 x 1024.