how much performance boost we get from dual/multiple core vs single core?

Netbooks and tablets are designed mainly to do very simple things such as browsing, chat, simple word processing and spreadsheets and things of that nature. The CPU and other chips they use are designed to be low power (to improve battery life) and their graphics and sound capabilities are basic. Dual core and 64-bit operating systems are only really of use to applications which are specifically built to use them and there is little or no benefit to most applications (indeed, the 64-bit version of Internet Explorer was markedly slower than the 32-bit version when run on the same machine and using 64-bit Windows). You don't say what version of Windows you are using but with only 512Mb of RAM, it is likely to be XP at the highest. You would certainly see a great improvement in speed by simply adding the maximum RAM your PC can support (which is probably 2Gb). 3D Rendering is probably the only application that you use which would benefit from a higher clock speed and a dual core processor but you would have to make sure that the rendering engine was actually built to support dual-core technology or it would simply be using only one of the cores.

You cannot compare netbooks and desktops - they are built for different jobs. Unless you need to be mobile there is no netbook that will come close to a modest desktop. My wife's PC has an old Athlon chip and runs XP. But it has 4GB of RAM (ok, not all used with 32bit) and XP flies! A 24" monitor is great with a 512MB graphics card (where a GPU does do some work unlike shared graphics) and big speakers and a good bass box gives quality sound. A Microsoft keyboard has a lovely touch and a laser mouse is streets ahead of a touchpad. It is connected to a laser printer and a colour multitask inkjet scanner/copier/fax which a netbook will still need. The big bottom line is that the PC didn't start out like that - it has been upgraded. If I thought a faster CPU would make any real difference I could plug in a faster chip. Try that on a netbook 18 months down the line! Bottom line. A netbook is great for what it was designed for.