Haha, I've also played with Cyrix's processors, the 486 (Which I think is the fastest 486's out there on per clock performance. Upgrade from my 486 33Mhz) their 6x86 processors (Friend) , their M2 processors (Computer shop I managed back in 1996), heck even the Via C7 processor (Sister's laptop) I've used them. (Cyrix was bought by Via and turned their latest processors into the C7 processors.) Good times....
Although performance wise, yes, they suck, except their 486's..... Still no match for a Pentium though.... And those 486's running on 133Mhz which AMD labeled 5x86.
Anyway back to the P4 VS Athlon era. The Athlon64 X2's still has the fastest processors....... If I remember correctly, their fastest Pentium D based processor is running at 3.6Ghz (Pentium Extreme Edition I think) it's a dual core with Hyperthreading, so they kinda end up with 4 cores total.
But still the 2.8Ghz FX-62 AMD Athlon processor still a bit faster....... About 20% at max. So AMD's still faster at that time, it was totally reversed when Core2Duo came in though......
Also, remember that it was AMD that first made a Dual Core processor, then Intel followed, making the Pentium D series........
However regarding single core. It was a very close fight I think. People could choose based on preferences...... I think it was the Pentium 4 Extreme Edition at 3.8Ghz and the AMD FX-57, which is a single Athlon at 2.8Ghz......
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Current PC specs :
AsRock 945GCM-S
Pentium Dual Core E5200 @ 2912Mhz (233x12.5)
Palit Radeon 4850 Sonic 512MB gDDR3
2x2048MB Kingston DDR2 800Mhz RAM