As far as I can tell, the BP6 w/ dual Celerons at 500+ is still about the best bang for the buck. About $300 and you only need one stick of RAM and one power supply. Assuming that dual C500's should be able to go to 562 X 2 fairly easily, you can expect about 3.3Mk/s. I had a Dual 600 ('til I sold it) that was cranking out 3.4Mk/s... it's still cracking, but under a friend's name (on our team!).
I think the next big bang for the RC5 buck will be the Duron. If one can get one overclocked to the 900-1Ghz range, you'll get around 3.05-3.4 for around $200.
The next best thing might be if you could get the converter to work for the BP6 and go with dual 700's at 930. It would run about $600 and get about 5.2Mk/s. With the and coming Duron though, this wouldn't be a very cost effective setup.
What I'm hoping for is a low cost (ala BP6) dual Duron board. MAN...can you imagine getting a setup for around $300 or less that will pump out between 6 and 7 Mk/s! I'm drooling!
Joe