Hi,
I've tried google-ing for some information on my RAM, but it doesn't seem to help much.
I'm trying to identify what type of RAM that I have, and what the stick that I am adding will need to have in common with the old ones.

My motherboard has 4 SIMM RAM slots. Two are in use, each with a 512 MB stick.
I'd like to add a third 1 GB stick, and I'd just like to make sure that I'm buying the correct type.

The model is:
512 MB IRx8 PC2-4200U-444-11-AI

I recognize PC2 as a type, but I am unsure as to whether the RAM I am adding will need to have more in common.

Also, on my motherboard, the two SIMM RAM slots being occupied are blue, the two free ones are black. They seem to have the same number of pins, but I am curious as to whether that indicates some sort of difference in the socket types.

I have uploaded a picture of one of my sticks aswell.

Hello and welcome to PC Help Forum.

The issue would be what is compatable with your mainboard?

What is make and model of PC?

Or what is make and model of your mainboard?

Much quicker googling them two.

You could do it this way:
Go to Crucial.com in Internet Explorer (will not work in Firefox, don't know about Opera) and click on "Scan My System", it will tell you what kind of RAM you need as well as other things.
Crucial.com Link

There are also a variety of programs that will tell you this:
Everest Home
|MG| EVEREST Free Edition 2.20
SIW (System Information for Windows)
SIW | Download
Belarc Advisor
Belarc Advisor - Free Personal PC Audit
Look In My PC
FREE PC Profile, Audit, and Diagnostic Report Utility

Just to name a few...

I have checked out Crucial.com.

It says that my model supports DDR PC2-4200, and DDR PC-5300.
Maximum 1 GB per slot,
and it says that the system performs best using pairs of the same type.

I don't really want to ditch my old RAM and buy 2 GB worth of 1 GB RAM, I'd much prefer to simply buy a third RAM chip. But how much performance am I loosing this way?

Also, I am checking online stores for this PC2-4200 RAM type, but all I see is DDR1 or DDR2, is PC2-4200 one of these?

PC4200 is DDR2
The PC#### refers to the speed of the RAM, the higher the # the faster it is:
DDR1 is as follows:
PC2100 - 266mhz
PC2700 - 333mhz
PC3200 - 400mhz

DDR2 (This is you):
PC4200
PC5300
PC6400
I'm sorry I can't provide you with the speed for these but I don't know them since I use DDR1 ram
If your motherboard is dual channel then it is in your best interest to use RAM in pairs, what that does is double the speed, like on a dual channel motherboard with PC3200, if you had two identical sticks it would write at 800mhz as opposed to 400mhz with three or one. If your motherboard isn't dual channel then don't bother but since your board has 4 slots I'd guess that it is, in which case you'd be better off with two (or even better off with four) identical sticks of 1gb PC5300 ram, that would lead to optimal system performance.
If you were to get a 1gb stick then your two 512 sticks wouldn't write at dual channel speeds anymore, your PC wouldn't be slow by any means at all but it wouldn't be as fast as it could be

If you post what your motherboard is I'd be happy to recommend some memory for you.
To get this, download Everest Home from here: |MG| EVEREST Free Edition 2.20, install it and once it's fully loaded, click on "Motherboard" then click on "Motherboard" again on the second page, then post what it says next to "Motherboard Name"