there is no reason buying a high end gfx card in macs, cos these gfx cards are meant for games, & Mac hardly got games. Even the GMA 950 or upcoming GMA X3000 is enough to run MacOS X UI. Apple should switch to Intel onboard graphics now, atleast this will cut cost. They support full openGL 2.0 in hardware anyway which is what Aqua is based on
On Mac pro, the story is different, it's a workstation, so quadro or FireGL are required. Right now mac mini comes with onboard GMA 950, iMac comes with ATI graphics which is gaming grade, while Mac Pro comes with 3 options only, & you cannot install any other card other then these 3. Check apple hardware page to find more
For HD, you have to buy from Apple, the Mac Pro comes with custom HD connectors, which although are SATA based, but are not like the general SATA wires or pwoer connector (wonder why). iMac & mac mini...well, they are hardly user upgradable, so again you will need to call apple service, & they won't let you put in your own HD
Unlike PCs,you cannot just go in the market & buy a hardware if you want to install in Mac, there are huge compability issues with firmware & motherboards in Mac. Normal PC components other then those not requiring a driver, don't work. Such as RAM or HD