Bad question up in this house!
If anyone replies, they'll tell you either a 1700+, 2100+, or 2500+ because they are all excellent overclockers.
Your board seems to support up to the 3000+ with with the newest BIOS running. If it was me, I'd go for either a 2600+ or 2500+, depending on if you plan on O/C'ing or not. If you plan to O/C, go with the 2500+, because people have easily reached 2.2 Ghz on cheap air cooling. If you don't plan to O/C, go with the 2600+, because the extra 200 Mhz will help you more than the doubled L2 cache on the 2500+. That would definetly make you a happy camper.
The best thing to do, would be to wait until the Athlon64's come out and the AthlonXP and AthlonXP Mobo prices drop a tad. Then you could go out and buy a better overclocking motherboard like the Abit NF7-S, a 2500+, and a nice HSF to throw on top of it. Then O/C and be a happier camper.
Good luck, hope this helped.