Dear Tahir
There are many good options for investment in Islamabad it would be more convenient if you will define your budget lines and approximate time lines.
If you want a long term investment then you can invest in F-16 B-17 and some other D sectors but it will take minimum time of 3 to 4 years for its extreme boom.
If you want to invest for short time like one year then you can search on Bahria enclave F-15 G-15 some part of G-14 and E-12. in F-15, G-15 and Bahria enclave you can get a good return within one year however in G-14 and E-12 there is a possibility of 2 years.
There are some other options also available but it will be clear after the things I asked.
How to know the exact price of plot this is a bit tough procedure ideal phase is to visit Islamabad and meet some dealers and also any of your references. After that Zameen is a good forum to find general price of a plot.