Get the fastest CPU you can, and go with 1GB of memory. It's cheap and there is no such thing as too much memory.
There are a few sites out there with quality coverage of the latest and greatest, but try
Anandtech - there is a slew of articles detailing the latest video cards there.
Currently, the Radeon 9800 is the performance champion. So, if you can wait and have no other need for a hotrod games machine, hang on until Doom 3 is actually out would be my suggestion as well.
Since we're talking Doom 3... check out that top article on the NV35 (the name for Nvidia's latest upcoming chipset)... they have a performance test using a version of Doom 3 in there. Here is a
direct link into mid-article where the comparison starts.