OK, let's think about things a minute: you can push the bus speed higher on some boards than others, and some boards allow you to "lock" the PCI and AGP bus speeds. Now, you want a MINIMUM of 133MHz bus (Intel multiplies their number by 4 because of QDR technology).
The 2.4 with 400 bus would have to be overclocked to 3.2GHz to reach the 533 bus clock of 133MHz. Not likely to happen. Faster processors will limit you even further: a 2.6 with 400 bus would have to reach 3466MHz, which is EVEN LESS LIKELY to happen.
So you go with the faster bus speed processors. Let's consider the 2.53, which runs at 19x133 (intel still calls 133Mhz 533). Well, if you were to bump that speed up to 150MHz, you'd have 2850. Not too shabby. At 158MHz, you'ld have 3GHz. Most boards will do at least THAT much.
Considering things like heat and max bus speeds for various boards, the 2.53 looks pretty good! But 500MHz is a fairly mild overclock. I'm sure you could do a bit more with good cooling and the right board.
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