The board only support a front side bus (FSB) of 200/266Mhz.
The 2600+ came in three flavours click here(1224) two are 333Mhz and one 266Mhz.
From what you have said it seems likely the one you have is a 2.08Ghz 333Mhz which as you can see in the table shares the multiplier of 12.5 with the 2000+ - hence that is why it is only being seen as a 2000+.
I think it unlikely that a BIOS upgrade will up the FSB to 333Mhz. A new mobo may be the only way to get the full potential of this chip.