If you plan to upgrade your PC for DOOM 3, you should check AnandTech's quick CPU performance comparison:

The first thing you'll notice is that the top of the chart is dominated almost exclusively by AMD Athlon 64 and Athlon 64 FX processors. Even the Athlon 64 3400+ manages to outperform the almighty Pentium 4 Extreme Edition 3.4, not to mention that the FX-53 distances itself from Intel's fastest by no less than 18%.

Making our way further down the chart we see that the Athlon 64 3000+ is quite possibly the best buy for excellent Doom 3 performance, weighing in right between the two Extreme Edition processors at less than 20% of the cost of those chips.

Next we have all of the Pentium 4s that manage to offer middle of the road performance under Doom 3, although we do see the K8 based Sempron 3100+ wedged in between the Prescott 3.2 and Prescott 3.0GHz CPUs.

Finally at the bottom we have the Athlon XPs as well as the lonely Celeron D, which is barely saved from a disappointing last place showing.