DDR Memory

I saw a pretty good deal for a 22" LCD monitor. After convincing myself that I couldn't live without it, it was on its way. Between the time I placed and received the order, I thought I might as well use two monitors rather than just do away with my old one. I dug up a few old graphic cards and experimented setting them up in my pc. None seemed to work and no XP drivers are available. After much time, I pulled my AGP card and booted on the old pci one. Seemed to work okay so I decided I could figure it out once the new monitor came. Nope, I couldn't figure it out! After playing around I decided it was much easier to order a new video adapter with dual outputs. While I had my computer open I noticed two empty memory slots. I thought I'd ought to order a couple of 1gb memory sticks to make me think my computer was running faster. While trying to figure out what was the cheapest, likely reliable, deal on ebay I learned that not all memory is the same. Some are high density (cheaper) and will not work with all motherboards. Ugh... I looked through what little information I have on my pc and could find nothing regarding density. I tried to read the SIMM's in my pc but my magifying glass was simply not powerful enough. I was thinking of pulling one out but then remembered the last time I tinkered with a working computer. So now I've decided that my computer runs fine the way it is and I am going to find something else to amuse me for tonight. How'd I get looking for memory from seeing an ad for an LCD monitor?