Asus only? I suppose tis best to go with what you are familiar with, and Asus are decent boards. I just bought a comp recently and went with a generic board - ECS I think - works great, there doesn't seem to be much difference between my Abit board, Asus board, and the new ECS preformance or features wise (well, the ECS has C-Media-notworthbotheringwith sound but..) motherboards are getting closer and closer together, giving no distinct advantage with one chipset over another, unless you need certain over-clocking features.
Ok, here we go:
Note that Athlon's seem to run much hotter than pentiums, as such I reccommend a good fan.
< Fan Reviews >
I found this to be helpful. You'll also want some Arctic Silver 3, naturally, to get the best heat transfer between the heatsink and the cpu.
CPUs: AMD, instead of going from K6-2 to Athlon to something else, used the popular name Athlon with extensions. You basicly have your different types and you went right through the list.
Duron: Cheaper, less on-die cache. been a long time since I've researched one of these. Slowest, not the best you'll find for games - compare it to Intel's Celery. Don't go with this unless you are really buget tight or don't need anything but internet/word processing.
Athlon (Classic): Don't consider this, few problems with the chip AMD solved later. Less features.
Athlon (Thunderbird): 800-1.4GHz I think. Fixed the problems with the orig chip, less heat, new features. Went out of date.
Athlon (Xp): This is the current, the only one you should look at getting. Naming scheme is weird - 2200 = 1.8GHz, 2100 = 1.73Ghz, ect. Xp's use a slightly different structure, allowing them faster clock speeds. These get good reviews every second place you look.
Ahtlon (MP): I think... athlon Multi-Processor. For servers and heavy load computers. To expensive.
As for what to buy: Since you want DIMM and DDR... Sis735 chipset will do, I'm not really farmiliar with any other chipsets that allow DDR/DIMM without slowdowns. My ECS has the Sis, works great. I'll have to check and see if Asus provides a like motherboard. For the processor, the best bang for the buck IMHO is the Ahtlon 1.73 (Athlon 2100). Its priced usually 25-50$ less than the Athlon 2200, and "only" 70mhz slower.
www.newegg.com provides competitive prices, and *perfect* service for them.
www.pricewatch.com for price compares, and
www.resellerratings.com for reviews of dealers.
Hope that didn't just repeat what you already knew, will research motherboards later.