If you had to choose between those, then the 6600... if you have to choose any AGP card, then +1 to rambo, there are better options available for you to pick from.
If those are your choices, then the nVidia 6600 is the better choice. The X600 is a PCI-E card so I take it you have a board with a PCI-E slot. If that's the case then go with something newer. Heck even the low cost Radeon 4550 or nVidia 8600GT/9500GT would be much better.