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The XP chips (XP stands for "extreme performance" and has no relation to Microsoft's Windows XP) debut at speeds that range from 1.33 to 1.53 GHz, but they're labeled according to a rating system that, AMD says, more accurately reflects the chips' performance. The new scheme is part of AMD's fierce battle to end the megahertz myth--namely, "The more the megahertz, the better the PC." It's the biggest marketing gamble AMD has taken in some time.

But regardless of whether this chip naming system pays off, the first Athlon XP machines seem top-notch.

In our tests of two preproduction systems based on the Athlon XP 1800+ (1.53-GHz) chip--the $2750 Polywell Poly 880XP-1800 and the $3049 Xi 1800+ MTower SP--both PCs outperformed a comparably configured 2-GHz Pentium 4 Dell machine (priced at $2919) on our PC WorldBench 2000 business-application tests and on several other measures.

Savvy PC shoppers already know that megahertz ratings don't reveal which processor makes a PC run fastest: Not until Intel pumped the P4's clock speed to 2 GHz did business systems using the P4 match the performance of PCs using the 1.4-GHz AMD Athlon, on most jobs. And if you're shopping for a high-end desktop PC today, you definitely have more than megahertz to consider. Looking ahead, Intel will also soon roll out some highly anticipated new P4 chips. However, analysts say that now--and even in the coming months--most value- and performance-minded buyers will find AMD Athlon XP PCs tough to beat.

Inside XP Ratings

The Athlon XP processor uses the same chip core, code-named Palomino, found in Athlon 4 notebook chips. An XP CPU uses about 20 percent less power than the original Athlons and boasts design changes that speed information to waiting applications. It also has 3DNow Professional, which adds 52 new 3D and multimedia instructions and makes it compatible with Intel's SSE instructions.

Some Athlon XP systems may get a boost from NVidia's new NForce system chip set (see "May the NForce Be With You"). But the most debated aspect of the Athlon XP will no doubt be AMD's chip-performance ratings.

The ratings rank XP chips based on their performance relative to that of the most recent Athlons. For example, the fastest new chip is the XP 1800+, which according to AMD performs like a 1.8-GHz Athlon--though its true clock speed is 1.53 GHz. AMD also asserts that PCs with an XP 1800+ should run faster than comparable 1.8-GHz P4 units, a claim borne out in our business-applications testing.

Likewise, AMD claims that the Athlon XP 1700+, which runs at 1.47 GHz, delivers more speed than 1.7-GHz P4 systems; the Athlon XP 1600+, which runs at 1.4 GHz, outdoes 1.6-GHz units; and the XP 1500+, which runs at 1.33 GHz, bests 1.5-GHz systems.

Athlon XP system configurations will cite the chip clock speed, but not prominently (see "AMD's Ratings Gambit"). You'll see the AMD term "QuantiSpeed architecture" on XP system configurations, too. The term covers various XP design changes that improve performance, such as an enhanced floating-point unit and the CPU's new abilities to prefetch some data and to successfully anticipate some upcoming requests.

Clearly, AMD can't convey such improvements through megahertz. They affect a less familiar measure of performance: the number of small jobs a CPU can do in a clock cycle. While that number varies widely depending on the app, there's no question that the Athlon XP finishes more work per CPU clock cycle than the P4 does, says Kevin Krewell, senior analyst for MicroDesign Resources.

Still, AMD's rating scheme may prove a tough sell. "When consumers see MHz in fine print, they may be confused," says Krewell. Moreover, this is a temporary metric: AMD plans to unveil a new rating system with industry partners, perhaps in 2002.

Other performance-rating efforts, from vendors such as Cyrix, have failed--in part due to lack of credibility, as companies played fast and loose with the equivalencies. AMD, however, plans to use an independent firm to support its claims. And it can back up its rating system with a solid product, Krewell says.

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