Thanks @ignatzatsonic and @Big Tom for your inputs & suggestions. I checked and found from Acer as well as RAM manufacturers (their vendors, actually) that Transcend make 4Gb modules are compatible and available at a good price (DDR3, 1333 Mhz) so will buy these, though it would make both my old RAM modules redundant.
@Big Tom, how do I check if all the CPU cores are enabled on my i5-based notebook ? I noticed in the Task Manager that it shows the usage graph for four cores, so I assume all four cores are enabled / active.
BTW, noticed that my Chrome browser alone takes over 1Gb RAM (as noticed in Resource Monitor) as it has some 15 odd tabs open. But, FF9 with about 8 tabs takes just 250Mb or so.