The PR number mistyriously matches the Celeron. The PR numbers to the AthlonXP used to be base, well pretty loosely, on the P4, but towards the end they were just pulling numbers out their buts. The proccessor would not ramp up and Intel was leaving them eating dust. The Athlon64, on the other hand, lives up to the PR muber MUCH better.
You could overclock it if you wanted. 2Ghz should be easy, even with stock cooling and voltage. You might see 2.2Ghz or more if you are lucky.
The should be something in the BIOS to let you adjust stuff, but if you take it in small chunks the system will just become a bit unstable in some applications. I used to use Quake3 and 3DMark2001se to test stability. Some use Prime95. My new one is Doom3 and 3DMark2005.
If you have not seen them, I'd download 2001, 2003, and 2005. They look pretty cool. They also give you a 3D score at the end for how well your system performed called ORB. Some people drop image quality to raise speed, so you might be a bit lower than the average for a similar system. The last 2 are very video card bound, but the 2001 likes a good CPU too.