You have 2 main routes. I also have a BX6 r.2
1) I upgraded my Celeron@450 to a P3 600E. Upped the FSB to 124 (limit of my then PC100) and got 744Mhz. Upgraded my RAM to PC133 and now have 800Mhz. P3 600E to P3 700E range offers a good route with max performance in overclocking to 800-900Mhz.
2) Upgrade to a Celeron 533A or 566. No need to change RAM and 100FSB also means no problem with AGP. Slightly lower performance route in the 850+ Mhz range with overclocking. Cheapest upgrade.