the new one is awesome dude, but it doesn't work with 5.1 surround sound, well not on my pc, have to use quadraphonic.
cod3 was good online too, but to be honest i think most people will have moved to cod4, but there should be plenty of people playing cod3, because like you've said, their pc may not be able to play the new one, so have to stick with cod3
you also have to bear in mind the cod3 was not made by infinity ward, they made 1,2 and 4, 3 was a different company, but it still is good, on my 360 at least
cod2 is great, search and destroy in the markets was always my favorite
its up to you, i think you can get both games relatively cheaply, but cod3 will need a better pc as it looked good on the 360 when it came out