Same here - all subs i made i used air or water cooling at the board. I experienced the same, that it heavily depends on the CPU (sub-0 on the cpu helps to push the fsb limit of the CPU some Mhz further but no wonders from my testing).
I can't remember my exact BIOS settings but i usally tuned GTL's a bit. There is a great article/Thread somewhere from that time which helped me alot - maybe you still can find it by "googling" (i sadly can't remember the page ). It was made for FSB OC the S775 Quads but also had some nice finds for Dual-Core OC.
Other than that i didn't do anything else then tweaking voltages and cranking up the frequencies. What i also can say is, that not all Boards like all straps. I had a Board which didn't liked 266er or 400er Strap but ran like hell with 333er. It also helped sometimes to lower the performance level to 12/13 and twister to auto at the same time.
If you have - try Micron D9GTR or D9GTS RAM's - always worked best for my REX's.
Good luck to you and i strongly agree: REX and S775 was the most fun!