Somebody tell me if this is bad form, I don't mean to hijack, and I understand I'm not the first in line.

I've tried PC100 in the Dell that has PC66 in it now and it didn't work in the Dell but it worked in the etower I took it from. Could I be doing something wrong? Or are there other part #* I need to find?

Dell = 3 slots, currently has 1 stick of 128 mb PC66 SDRAM. I'd like to have 2 sticks of 128 for this one.
EManchine 1 = 2 slots, has a stick of 32 mb PC100 and a stick of 128 mb PC100
EMachine 2 = 2 slots, both 128 PC 100 SDRAM.
I only want to upgrade one of the EMachines, and I'll RAM the heck out of it, so I want 2 of the biggest sticks I can get that will fit it.

ok, I have so far

3 - pc100, 128M, single sided
2 - pc133, 256M, double sided (32x64)
2 - pc100 ecc, 256M, double sided, cl3

What is ecc?

Error Correcting used mostly in servers or high end workstations.

I've still got two old "socket 7" back-up computers that use the pc100 memory sticks. I've read that the pc133 sicks may not work -- may have to do with the cl# -- the lower the number, the better the chances it will work in older computers.

At this point, though, having diddled around with various sticks some, I think it'* sort of a crap-shoot. A stick that won't work in my one board will work in the other. And the motherboards are near identical. So you just have to try, and pass them around. You do have to pay attenton to what sticks you have in the shorter slots (when existant, and I forget their names) as they can interfere with the longer sticks. Even though the directions tell you otherwise.

For some reason or other the 256mb sticks and larger get very precious on eBay.

UPDATE

3 - pc100, 128M, single sided
2 - pc133, 256M, double sided (32x64)
2 - pc100 ecc, 256M, double sided, cl3

2 - PC100 ECC, 128m, cl2, double sided
2 - pc2100 , 256M, single sided

I am sending Andrew the pc133 dimms, we will see of they work

Regarding the ECC (error-correcting) type sticks:

These were intended to be used in computer "servers" of the era. My understanding is that they can be used in "home" computers, but since one bit is devoted to error correction, you lose 10-15% of the available memory for yourself. Not a big deal if you can get a hold of 256 or 512 mb sticks.

It'* possible, though, that you have to have "all" ECC or "no" ECC in the set plugged in. I'm not sure of this, but it'* another thing to watch out for as you're trying out sticks. If this turns out to be the case, it would be good, of course, if BC members could sort them out accordingly amongst themselves.

Alot of what memory works is dependant on the chipset being used on the motherbd and how much memory support the manufacturer implemented.
Another issue is the chips that make up the ram and how well they 'work' with the chipset.

All chipsets have configuarion limitations.
Here are some older Intel chipsets. Note how some, like the 810/815 series, can only go to 512Mb. There is no way around it.
http://support.intel.com/design/chip...ture/index.htm

Heres some the 430 & 440 Series. Who remembers the BX chipset?
440/450: http://www.intel.com/design/chipsets/mature/450_440.htm
430:http://www.intel.com/design/chipsets/mature/430.htm

These are just Intels Chipset, theres also Via, SiS, Ali, Opti, Cyrix... to name a few

Ideally you want to find the owners manual or the info on the manufacturers website on what works for that board.