In the context of comparing outdated processors (yes, even my Athlon64 3200 is outdated) to current offerings, no, clockspeed didn't matter nearly as much back then as it does nowadays. The Athlon vs P4 debate is all about L2 cache - the Athlons had plenty, the P4s had next to none. That's why a 2GHz AthlonXP would outperform a P4 running at 2.4GHz. The AthlonXP had far more cache, so it could waste less clock cycles on cache clears and subsequent accesses from RAM.
Nowadays, both AMD and Intel processors have plenty of cache on them, so the only major differences in performance are the FSB (Intel) vs HyperTransport (AMD) argument, shared L2 cache (Intel) vs dedicated L2 + shared L3 caches (AMD), and the DDR2533 vs DDR2667 vs DDR2800 argument (DDR2667 on a Core 2 actually results in lower performance than DDR2533).
Try playing Supreme Commander on a single-core machine. It's no longer "useful," it's "mandatory."