Update:- Up and running with 40GB of RAM. The manual says it's OK to mix different sizes as long as config is the same (these are exactly similar), so I slotted in 2 of the old modules. Ran a Dell Diagnostic test on memory and passed all. Makes a huge difference to my spreadsheet work. Scat, a big thank you for your kind advice that allowed me to avoid being ripped off. The Dell partner quote for the RAM was $1,537, I got it done for $462 or 30%. I don't get the Dell warranty but I do get the manufacturers warranty. Incidentally the OEM for this IBM RAM is the same outfit that made the RAM in the original config.
Good thing you checked around. While Dell sometimes has some really good deals (especially on base configurations), they are notorious for excessive markup on upgrades. By making sure to buy the same voltage and timings, you're able to use your 4s with your 16s. I suspect with different size RAM sticks, your rig is operating in "dual channel" mode. For best results, the 16s should be in one bank, the 4s in the other. That is, DIMM slots populated with 16s in 0 and 2, while 4s are in 1 and 3. Check the manual to verify. There are about 10(?) major chip makers. All are made to the same specs for a given model and are about equal quality. Good luck!
I am not sure you can use that much amount of ram if the model of notebook is not top of the top anyway really. So I am leaving it all to you really. Please be sure to ask manufacturer before spending crazy money on RAM really. It's not something you really want.