This topic can be incredibly in-depth, but I'll try my best.
To start, Intel never made a P3-400. The slowest P3 you can get is 450. And since there's no common speed between any particular P3, P2 and Celeron, there's no truly fair comparison.
But if there were, the P3 would be slightly faster than the P2, due to it's added instructions. That would be about the only difference between that speed P3 and P2, since P3s that slow were still based on the Katmai core.
Until the release of the Coppermine core Pentium IIIs, there were only marginal speed differences. The Coppermine reduced the L2 cache by half, but doubled the speed of it.
A quick matrix on CPU differences:
P2 - Katmai core, 512k L2 at 1/2 core speed, 100MHz Front Side Bus (FSB) speed, .22u process
Celeron - Katmai Core, 128k L2 at full core speed, 66MHz FSB, .22u process
P3 Katmai core (500MHz and lower) - 512k L2 at 1/2 core speed, SIMD instructions added, 100MHz FSB, .22u process
P3 'b' - Katmai core, 512k L2 at 1/2 core speed, 133MHz FSB, .22u process
P3 Coppermine - 256k L2 at full core speed, available in 100MHz FSB ('e' coppermines) and 133MHz FSB ('eb'), .18u process
Celeron II - Coppermine core, 128k L2 full core speed, 66MHz FSB, .18u process
Laptop Recommendations
- Make sure you buy the largest TFT screen you can afford. Don't skimp and go for passive matrix, since your eyes will be teary by the end of the day.
- Be more concerned with the amount of RAM than the speed of the CPU. Most things you'll do on a laptop will be more RAM intensive than CPU reliant.
- Don't expect for the laptop to be a replacement for a good desktop machine. People normally think that they can buy a laptop and take it with them while still using it at home, and that justifies the extra cost. Laptops are built for portability, not speed or comfort. If you truly need the portability because you require a computer on the road with you, then fine. But if you just need a mahcine and like the idea of having a laptop, you'll be dissappointed. For the same price, you can get a very nice desktop machine for much less money that will outperform any laptop you can find.
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On an long enough timeline, everyone's survival rate drops to zero.