Advice on a specific upgrade please...

Crucial.com should guarantee it's ram to work; may be worth spending a little extra so you have no return hassles. All desktop ram is non ecc; ecc is for servers only and should be listed somewhere in the specs.

So what about the PC2-3200/4200 on the specs from Dell's site vs. the PC2-5300/6400 on the recommended from Crucial's site?

I'm not overly concerned with any return hassles. I will most likely get the sticks from newegg, and they have a 30 day return policy. I think I understand the buffered vs. unbuffered and ecc vs. non-ecc...and most sticks I've been looking at are what I need for those requirements.

So, basically...I need to understand what the difference is between Dell's spec sheet pc2-3200/4200 and crucial's recommended pc2-5300/6400.

THe PC2-3200\4300 are different speeds of the RAM modules ie the 3200 are running at 400Mhz. the 4300 runs at 533Mhz. - the others you list PC2-5300\6400 are newer modules that run at 667Mhz. and 800Mhz. and are newer than your MOBO was designed to support (unless they have done a BIOs Update since the system was released - Judging from this page you listed ( http://support.dell.com/support/edocs/systems/dim4700/s... ) - your MOBO only supports the older 400-MHz (PC2-3200) and 533-MHz (PC2-4300) DDR2 unbuffered SDRAM non-ECC so unless you verify that the others are compatible I would stick with them !! - Also are you completly replacing the memory or adding to the existing ?? - If adding more then you need to find out which speed is already in there and get some that match it (mixing speed will sometime work with the faster modules slowing themselves down to run at the lower modules speed but can cause stability issues so best not to if possible !!) - Also if adding to existing modeules be sure to get ones that use the same voltage as the old ones (the MOBO can only supply the same voltage to all of the RAM so you need all of them to run at the same voltage - again for stability reasons and again mixing will sometimes work but best not to chance it !)