I will give you a bit of an update...
1900+xp which is actually 1.6ghz beats p4 2.0ghz in most of all of the benchmarks
1800+xp which is actually 1.533ghz beats the p4 2.0ghz in majority of benchmarks
1700+xp which is actually 1.466ghz is about even with p4 2.0ghz
1.4ghz tbird is about equal with 1.8ghz p4
IMO, the athlon is using the number you see (ie 1900+ xp) as a way to compete with the marketing scam intel is trying to push on the consumers. They have highest clock speeds but amount of instructions done in clock cycle is the lowest. They lack any real fpu power like amd and are counting on sse2 codings to be done by software companies to make up for that shortcoming...1 year gone and most are still waiting to see this realization of speed.
check out these sites...
www.anandtech.com
www.tomshardware.com
www.techreport.com
They will all proclaim xp "is current" champ...
What is better for you may be in the programs you run so pay attention to that and the percentage difference in some of the test as the difference may be not even noticeable.