Excellent.
Well according to Dell's site you need DDR 400 Mhz RAM modules.
Dell : Memory : Accessories : Home & Home Office

These are relatively easy to get hold of and quite cheap as well.

Will this one work?

1GB PC2700 LOW DENSITY DDR MEMORY 4 DELL DIMENSION 2400 - eBay (item 200270479830 end time Jan-01-09 00:41:55 PST)

Never mind, the one I linked to is only 333mHz; I found a 400 mHz from the same seller. Thank you so much :)

Alrighty! I just received my "Dell validated" 1GB, 400mHz RAM in the mail... now what? Lol

Is it a "plug & play" so to speak? Do I pull off the cover, insert it(with the computer off, of course!) & I'm good to go?

Yep, that's about it.
You may get a RAM warning, but READ what it says and go from there.

Make sure you earth yourself so as not to damage the memory module.
Also remove the old 128 stick as well.

Okie dokie; got it in! Would have saved a lot of time, had I not experienced a blonde moment and forgotten that the side panel has a "release lever" lol

It seems to be faster but could still use some help. For example: I turned it on, Windows loaded and asked which profile I would like to use. I scrolled over the name and it took 8 full seconds for the mouse arrow to change to a hand. Do you recommend I try everything(defrag, CCleaner, etc.) all over again?

To run event viewer, go start/run/type eventvwr.msc, then take a look at both the applications, and system errors. You might see something that happens at the same time as your windows logon. Only worry if it's an error.

Another thing you might try is downloading and running Process Explorer, and looking for processes that are hogging your CPU (event viewer will give you a static view of errors that have occurred, process explorer will give you a dynamic view of what's happening in your CPU).

All of the most recents errors in the Applications file are described as IE "hanging" and the last one occurred yesterday at 8:55am. I assume that's just when it temporarily freezes, which it hasn't done quite yet since I upgraded the RAM last night around 5pm.

There hasn't been an error recorded in the System file since 12/19; that was to tell me my AVG E-mail scanner didn't load quick enough.

I also downloaded Process Explorer; doesn't seem like there's anything unnecessarily running or taking up a lot of space.