Personally I'd drop over to Azzo.com & grab a C566 & a decent Slocket. Total should run you under $140, plus $20 or so for a Golden Orb. You'd have a pretty good shot at 850+ MHz, & it's cheap. You'd be able to use your existing motherboard & memory, just a nice & simple CPU swap.
If Durons were easily overclockable, they'd probably be a good choice. Right now, though, a Duron 700 costs $175 locally. And that's just for the CPU. You'd also need a new motherboard. Total would probably be $300-325. And frankly the Duron doesn't kick ass on the Celeron II as badly as people might like you to believe.
The Celeron's not the fastest thing out there, not by a long shot. But it's a damned good CPU for the money.
Viper GTS