Well, alot has been made of the various steppings, but the two that have proven to be the best overall are the week 10 year 2003 (0310) WPMW and XPMW steppings.
There was a thread comparing these two a while back, like 3 weeks ago, where a guy in Ontario got a shipment of both steppings and tested them out for max OC at his pc store (where he works).
His results showed that the WPMW on average overclocked a slight bit better than the XPMW, but that both were typically able to do 2.3ghz on regular air cooling (he used a Vantec Aeroflo if I'm not mistaken).
Personally, I ordered a WPMW over the weekend, but a buddy of mine just got the XPMW and is running currently at 2.08ghz with 1.5volt (default, which actually ends up being somewhere around 1.46volt on his Abit NF7-S rev2.00).
Either way, I don't think you can go wrong but most people here have been talking about the XPMW stepping... dunno why, maybe that's what most people bought at first
GL with whichever you choose!
Methusela