I recently bought a Celly2-850 and currently have it OC'ed to 8.5 x 124 = 1.054 GHz. I bought it as an experiment to see how much better it would be than a Celly2-600/900. I then used to Celly2-600 to replace my daughter's Cel-300A/450, which will replace my Cousin's PII-300. All the system run SETI@Home and I'm just trying to improve output. So far the results are great!
When I get back to my Corportate Apt. after the holidays I plan to try the new celly2 at 133 FSB. I currently have the retail HSF installed, might need to change that. Actually I hope it will still be running when I get back. The mobo is an ABIT BH6 rev1.0. The BIOS thinks it is an 800EB, but it still works. The multiplier is set automatically and I can adjust the FSB. This old board doesn't have many FSB options.
If your system supports it, you might try the 900MHz and OC it, instead of getting a 1.2 GHz.