have both moved to PR numbers, as Intel states "PR numbers are not indicative of processor performance", they're merely market names.
AMD's market numbers have a bit more sense, they don't directly correlate to a doubling of Intel speed, they're metric is an older Intel CPU, the Pentium 4 Northwood A (about 3 years old as of this writing). The PR number roughly correlates to that CPU's speed in MHZ, which means an Athlon64 3800+, is roughly a 3.8GHZ Pentium 4 Northwood A, still doesn't tell us much. with Athlon64 x2, they list the PR of a single core, and add X2 to the name, so the Athlon64 3800+ and X2 3800+ are both 2GHZ 512k L2 parts, but the X2 has two 2GHZ 512k cores, instead of one.
Now, dual core DOES NOT MEAN, dual the performance, it just means more operations running at once, which does increase performance, however it isn't a full 2.0x (if you want a further/more complex understanding of this, and don't mind the math, look up Amdahl's Law)
Now for a general bit of info on AMD and Intel processor nomenclature:
Sempron is AMD's value line, just like Celeron is Intel's value line
Athlon is AMD's mainstream line, just like Core is Intel's mainstream line (read futher for more on Core vs Pentium)
Athlon FX is AMD's performance line (soon to be replaced), just like Extreme Edition is Intel's performance line (these CPUs are targetted at enthusiasts who are willing to pay upwards of $1000 for a CPU, and are generally not worth the money to the average user)
Pentium is an interesting monkier, originally it was Intel's primary line, with Celeron added alongside Pentium II, Pentium continued through the Pentium D (dual core Pentium 4) line, and then more or less died from the market
recently a Pentium branded CPUs have appeared on the market, as low cost products, they are nowhere close to the actual Pentium 4 architecture, and are instead re-badged and upsold Celerons, their performance is below that of a Core 2, but above that of a Celeron D, generally speaking
The only CPUs missing from this are AMD's Turion (which is a mobile series Athlon) and Intel's Core Duo (very different from Core 2, its a mobile series chip though), anything else, is likely a server/workstation market chip (Opteron, Itanium, PA-RISC, POWER, CBEA, Alpha, MIPS R, X1, SPARC, etc) and wouldn't be practical for home use computing (Xeon is excluded, as its used in the Mac Pro, however its still a workstation CPU, not a consumer CPU, Apple uses it because its multi-CPU capable, allowing 8-core and 16-core systems to be sold)