Ok, if you're dead set on that 1.5GHz Celeron, look to see what chipset it has...
If it's got the Intel 915, you can buy a 400MHz FSB Pentium M (make sure it's one of these: 715/725/735/745) and pinmod that sucker to run a lot faster... pretty much you're going to trick the computer to run the 400MHz processor in 533MHz FSB, effectively overclocking it.
Like I took out my Pentium 730 (1.6GHz, 533MHz FSB), threw in a 725 (1.6GHz, 400MHz FSB), and pinmodded that (to trick it into running at 533MHz), which netted me a 25% gain in clock speed. Ended up getting 2.13GHz without a significant decrease in battery life.
Oh, and the "Centrino" thing... A "Centrino" is just a word Intel used if the computer houses an Intel chipset, Intel processor, and Intel wireless card. Say your laptop's manufacturer put in a FAR superior wireless card (i.e. an Atheros one)... the laptop is no longer "Centrino."
