General HP Desktop Questions

Sorry this may have been posted before. I was wondering, I may be able to aquire an HP Desktop Computer (Model Number: M7590N {http://www.futureshop.ca/catalog/proddetail.asp?logon=&langid=EN&sku_id=0665000FS10076548&catid=10607} ) for next to nothing. Is it possible to remove the power supply, processor, motherboard, ram, hard drive, and optical drive, (Buy a new video card, and case) and put it all together. Or will there be some compatibility issues? I have heard before that prebuilt systems are the hardest to update. I eventually plan to slowly replace everything {But not right away of course}.