what system are you planning to overclock on? Why only use PC2100 RAM? Also why did you mention the Athlon4? Except ebay I doubt you can get that anywhere by now.
If you were talking about the Mobile AthlonXP then you will want RAM that can do more than 133MHz. I'd go for PC3200. if you don't have a lot of money, generic PC3200 will be better than most PC2100 RAM.
For good cooling I'd go with a thermalright heatsink, for the price that you can afford
www.heatsinkfactory.com is one place to look for them, only remember you will also have to get a fan for these heatsinks.
For the CPU, again depending on your budget either the XP-M 2400+ or 2500+. 2500+ will be a better overclocker, but the 2400+ isn't bad overclocker either, and will OC better than desktop bartons.
Do not get a duron that is sub 1400MHz as these are Morgans and do not go over 1.4GHz on the most part. The new Applebreds (1400 and up) are just TbredBs with most of its cache disabled, so they overclock great, but they are locked, and not unlockable. They also run at 133MHz FSB, so PC2100 RAM will only be able to run it at stock.
what motherboard are you going to use for that system anyway?