If i have ddr rams, can i just replace my ram as ddr3 without any modifications?

You can not upgrade nor downgrade your ram from DDR1 to DDR2 or to DDR3. You are stuck with whatever your motherboard will support. You can however, install new ram cards that have more memory ie (from 512 MB to 1 GB). When you are installing new ram cards you simply unlock the old ones from each side. Then take out your old cards, and put in your new ones and lock them. No software is required for your new ram cards.