i am getting a HP Compaq CQ42-462TU laptop at very cheap price ($520), configuration below
processor - Intel core i5 460M @ 2.53 GHz
Chipset Intel HM55
ram - 3GB ddr3
but graphics are Intel HD 3000
Can I add any extra graphic card (ATi's mobility or nVidia's Go) along with integrated Intel graphics like they used to come with some higher models?
Doubt it! If the machine only comes with intergrated graphics, then it is very doubtful you can upgrade the graphics for a dedicated GPU. Check HP upgrade section for your machine but I don't think it will be possible.