right, I have an old motherboard that I'm gonna replace cause many of the capacitors are blown, but I'm confused about the memory- my old motherboard supported this:

2x DDR 333/266

my new motherboard's specs are:
2x DDR slorts, DDR DIMM 266 - 400Mhz supported

will the RAM from my old MB work with the new one? I've tried inserting it and it fits perfectly

Since they fit and specs are in range they should. It's important all memorysticks have the same specs for a proper recognition in bios/os though.

Yeah, that should be fine. Motherboards will usually support a range of speeds, it doesn't have to be an exact model. And you can also overclock (or underclock) the memory up until a certain point, so that may give you a little more leeway (although if the motherboard won't read the RAM at all, then you won't likely get that far).

again, should work like a charm

If you have two identical sticks, be sure to check the manual on which slots they should be in for dual-channel mode to work.

Somo mobos are slightly different. I've seen some Dell systems that look like they use the same memory as any other mobo of that type [forgot which model(s) but they are older Dimension model] but the notches in the memory between what works and what doesan't don't match up.

That is strange. RAM is standardized, meaning any DDR2 module should fit in any DDR2 socket. Not sure if Dell uses something proprietary, but that doesn't make much sense.

yeah, my dad had an old Dell Dimension, and it had RDRAM, an old forgotten standard that was launched by Rambus to replace DDR SDRAM, but turned out to be just equal or slower in performance. It wasn't proprietary to Dell, they just were the only ones using it on a larger scale.

nowadays, AFAIK, there is only one standard(DDR) and 3 different revisions of it

There was just a tiny difference at the notch.