PC2100 Registered ECC. That's what Dell sent me, and what I bought
more of. (I didn't check whether the motherboard is capable of using
higher speed RAM or not, I just assumed Dell must have sent me the
fastest RAM that would work.)
Their prices seemed to the lowest at the time, so I bought mine at
newegg.com.
I've ended up buying a large percentage of my computer stuff at New
Egg, they're a good vendor.
techbargains.com
is a good place to compare RAM prices. Sometimes
Crucial's
prices are actually the best but usually not.