Ok, first off, one of the best places to get details on the Athlons is off of
ArsTechnica. The boards there are especially helpful as these people are constantly up to date on product offerings.
The Athlon's in the market now range from T-birds up to the newest Barton cores. Most sites, such as Newegg and Googlegear give a lot of information about the specs of the cpu they're selling. Now the companies should tell what the core design is and whether it was made on the 18 or 13 micron process. You'll actually find that some of the older 18 micron Palamino chips, such as an 1800+ are more expensive than the newer T-Bred 1800+ which are made using the 13 micron process. The reason for this is purely supply and demand as some older boards can't handle the upgraded FSB of the T-bred.
Now all sorts of problems have arisen since AMD released two core version of the T-Bred. Since the packaging of retail and OEM cpu's don't state which core revision is used people ahve become a little paranoid about what they might be ordering. Although some of the places such as Newegg have been nice and will sometimes say which one you are getting, its often hard to tell. The only definitive way is to look at the chipset #'s on the back. You should ask the people on the Ars forum for what the latest designations are as I can't recall them off the top of my head, but I believe a U signifed the B revision.
As for how FSB and the clock multiplier work... the core speed of a cpu is determined by those to thigns alone. In perfect theory, the FSB is multiplied by the clock multiplier which yields the core speed. Thus, a 266Mhz FSB and a clock multiplier of 11.5 would yield 1.53Ghz, the setting of an Athlon 1800+ T-Bred with either A or B cores. This leads to the ability to OC the core by moving the FSB up in 1Mhz increments or moving up the clock multiplier in either .5 or 1 steps. In theory, an old Pentium with an adjustable clock could be put on a 200Mhz FSB with a clock at 3 and you would have a 600Mhz Pentium. The problem in the real world is that the chip would literally be burning. With the addition of more transistors to the core due to smaller manufacturing sizes allows for a higher core speed / heat ratio.
Some good places for information are:
ArsTechnica They have some very thorough blackpapers and their forums are filled with people who know a lot more about this stuff than I.
HardOCP One of the most definitive sites about overclocking.
TomsHardware This is a must read for tons of information and reviews of products.
Last but not least... Anand's own articles carry a wealth of information that deserves a read through.