Take into consideration that the P4 can't do SMP. To do that you need to Xeon which is closer to $500.
Also, just going to pricewatch and grabbing the cheapest CPU you can find isn't the way to compare prices here. Correct me if I'm wrong, but isn't the Opteron being billed as a server CPU? And for some reason power users latched onto it while waiting for Athlon 64. I guess if I wanted to look at game benchmarks, I wouldn't compare the Athlon to the Itanium. They're different processors for different uses. Let's look at some TPC results to see how this processor stacks up where it seems to have the brightest future - servers.
Dell  PowerEdge 6600/4/2.0GHz/32GB  78116.87 4.85 US $  4 Intel Xeon MP 2.0 GHz  
RacKSaver  QuatreX-64 Server 4P  82226.46 2.72 US $  4 AMD Opteron 844 1.8GHz  
Dell  PowerEdge 6600/4/2.8GHz/32GB  84595.22 3.84 US $  4 Intel Xeon MP 2800MHz  
Here's the only Opteron with TPC-C results listed, and the Xeon machines that are closest in performance. The benchmarks are bolded. Based on price/performance the Opteron slaps the Xeon silly. Looking at the all the results for the past couple years, the Opteron machine has one of the lowest price/performance ratios of any server. And the servers that do have a comparable ratio are all 75-80% slower than the Opteron.
Dell  PowerEdge 2650  20108.79 2.28 US $  
HP  HP ProLiant ML350G3-1P  19526.27 2.38 US $  
HP  HP ProLiant ML370G3-1M-1P  19718.01 2.44 US $  
HP  HP Proliant ML370G3-1P  19140.72 2.45 US $  
IBM  IBM eServer xSeries 235 1P c/s  18936.05 2.46 US $  
HP  HP ProLiant DL380G3-1P  18818.46 2.47 US $  
RacKSaver  QuatreX-64 Server 4P  82226.46 2.72 US $  
Dell  PowerEdge 2650/2.4/1P  16756.52 2.78 US $  
IBM  IBM eServer xSeries 225/2.4Ghz/1P  18077.98 2.79 US $  
IBM  IBM eServer xSeries 235/2.4GHz/1P  17559.31 2.99 US $  
Dell  PowerEdge 4600/2.2/1P  12579.04 3.31 US $  
If you want to compare AMDs latest and greatest to Intel, why not wait until they actually release the desktop version. Then if that sucks you can flame away. 
Edit: Granted, there is a vast amount of configuration that has to be taken into account when looking at TPC benchmarks including storage, clients, etc. But generally a company tests the configuration that is going to show them in the best possible light. If Dell could build a server to compete with the RackServer performance for the same price, they would.