I have played doom 3 on my old pc:
pentium 4 1.5ghz
512mb Sdram 133mhz
nvidia 5200xt
With this computer i couldn't even play on medium quality of course my videocard was way too slow. but i didn't upgrade it because i wanted to buy a new computer.
I have now pentium 4 3.2ghz HT technology
1024mb PC3200 DDRAM 400mhz
6600GT Pci_e 128Gddr3 ram
With this new computer and videocard i can play it even on ultra quality without slowdowns. For this game you really need a fast computer. This game is ment to be played at ultra quality but the most people don't have the computer for it to play the game the way it's ment to be. So that's why they have added low, medium, high and ultra quality. So do you want to play the game the way it's ment to be you need to get a better PC.