I bought RAM from crucial 5 years ago for my Dell XPS T550 PC. I also have a Dell Dimension 8300 system and bought from Crucial.com yet again to upgrade my RAM from 0.5GB to 1.5GB.
Also, your question is kinda vague. By ''best'' do you mean the best that $$ can buy or the best value? Either way, there may be some places that are cheaper than Crucial.com. For example, Monarch sold 2 sticks of 512MB Corsair brand for $105 while Crucial sold Micron brand (their) for $115 + $6 for NJ sales tax (both included free shipping). I ended up buying from Crucial site b/c 1) customer loyalty and 2) Monarch was kinda vague about what goes with what. Crucial tells you up front what will work with your prebuilt PC or mobo.
As for specs, I *highly* recommend you visit Crucial.com for 2 reasons. One, u'll see what's compatible with your PCs. E.g. choose brand for Dell, Dimension for model line, and Dimension 8300 for model, and you'll see a list of compatible types/specs of RAM, as well as other info on your PC, like how the 8300 has max of 4GB of RAM and 4 DIMM slots. You can do this for your emachines as well and by motherboard if you put together your own PC. Two, they have alot of links, FAQs, and guides that explain each spec. I don't know what's standard with RAM on the 8300, but i believe you'll want and Crucial will also confirm this as well.....
>PC3200 speed (which is 400MHz)
You can mix PC3200 speed RAM with PC2700 (which is 333MHz), but actual speed will underclock to the slowest stick. May as well get the PC3200 since due to the way production is, PC3200 may actually be cheaper if not the same price
>non ECC (Error Correction Code)
this is used moreso for servers
>dual channel RAM
You get better performance from buying a pair of RAM of half capacity each instead one whole. Dual channel RAM uses the extra bandwidth available from 2 sticks of 512MB as opposed to 1 stick of 1GB to gain better performance. The pair of modules MUST be the same speed and capacity. They don't have to be the same brand, altho it's recommended they are (which is the reason many vendors sell them in pairs as "dual channel" kits). They also have to be installed into the correct slots for it to work. Check your PC manuals for the RAM installation section. They'll tell u which slots to install to, label the slots, or indicate that it doesn't support dual channel
...and you can look up the rest yourself. When in doubt, just go by the compatiblity list. You may not buy from Crucial, but at least you'll have a better idea about RAM after 5 to 10 minutes of reading.