You can buy a slot-to-socket adapter for about $10, and a socket370 (FCPGA) p3 or celeron for under $100.
I wouldn't trust one of those $10 converter boards, there are lots of reports they don't work on many motherboards. You *may* need to have a converter board that offers its own voltage adjustment if your version of the P2B-F doesn't support the lower voltage of newer S370 CPUs and most of the $10 generic converters don't offer that.
I upgraded an older P2B (PCB 1.04) from PII 400 to 1GHz Celeron using the MSI-6905 v2.3 converter board and it will support PIII 1GHz, also. There is some BIOS problem with the P2B and 10.5x or higher multipliers, so 10x was the max I could go. Don't know if it applies to the P2B-F or not.
The brand name converter boards are becoming difficult to find. There were several on Ebay a month ago. I bought my MSI for $30 and the 1GHz Celeron was $60, add a $10 CoolerMaster S370 HSF for about $110 total with shipping.
Check this
CTechNet Asus page, it has a lot of info on the highest CPU capable on various Asus boards including which BIOS you have to use. It also has some info on CPU converter boards.