Amazon is perfectly safe to use and shop from, I have been buying from them for over 5 yrs and never had any trouble, especially when you purchase from Amazon directly and not a re-seller. Ebay is perfectly safe as well, just look for 100% feedback sellers and pay with paypal or thru eBay they both offer buyer protection, so if you run into a problem with the seller the'll work it out.
As anort3 said, DDR 400(PC3200) will work just fine. I have only run into an issue of using ddr 400 in a 333 board once and it was a laptop. Due to its design, it did not feed the needed voltage for DDR400. A quick SPD flash(ok, its a bit nerve racking, to ad to it, I hot flashed it) to the stick of memory and it was all good to go.
I have never seen issues like this on desktop systems.
If you are in Canada or USA, you can get pc2700 if you really want it here