Hey,when i install the 64bit version of the OS will i be able to get the full use out of my 3gigs or i should install at lease 4-5gigs memory?
As I stated before, DDR2 does not work well in uneven numbers. Your motherboard normally has 4 RAM slots (2 if it's a smaller board), numbered 0-3. 0 and 2 work as a channel, and 1 and 3 work as a channel. DDR2 can operate in dual channel mode which effectively doubles your memory bandwidth, provided you install it in the correct slots, and the modules are of equal specification.
My recommendation would be:
a) 2GB modules in the 1st and 3rd slots if you want 4gB.
b) 2GB modules in each slot if you want 8GB.
That will ensure that your RAM works at peak efficiency.