The SL2HE is an old 2.8V, 0.35um Klamath core processor, so the most you can hope to get out of it is about 300-333MHz. The good news is, its not multiplier locked, so you can change that. Currently its at 4.0 x 66MHz, so you can use any combination of multipliers and bus speeds your board offers to get it to run. I recommend trying 4.5 x 66Mhz, or even 5.0 x 66MHz. Depending on what your board offers, you could also try 75MHz or 83MHz FSBs. Good luck.
"In the computer industry, there are three kinds of lies;
lies, damn lies, and benchmarks."