1) There is your main memory (also called Real Memory or RAM)
2) There is your hard disk memory.
Increasing 1 above WILL speed up your computer. So if you have 512Mb, doubling it to 1Gb will speed up your PC (probably), or if you have 1Gb doubling it 2Gb may speed it up.
Increasing 2 above will probably NOT speed up your PC (unless your hard disk is totally full) and then all you need to do is delete some files, and that may speed up Windows a bit (but maybe not a lot).