So many games to recommend, I am having a hard time keeping my list small.
I think I compiled a list like this before so I'll dig that up and post back :)

Off the top of my head, I would STRONGLY suggest the extremely under rated and over looked AQUARIA.
It is an indie game that is like a mix of Metroid, Ecco the Dolphin, and Geometry Wars with an amazing story. I am pretty sure the specs aren't too demanding, but I may be wrong though I haven't tested many games on old PCs in awhile. I'll post back with a better organized list  ;)

Heroes of Might & Magic III
Civilization II

That should keep you busy for months !

Stronghold Legends - An atmospheric RTS with lots of resource management in a medieval/lore setting.

Made in 2006 but will run well and look great on your comp.

Already Played Doom, didn't get into Doom II (I have the actual CD from id with Ultimate, II, and Final), played Duke Nukem. I'm seeing a lot of RTS recommendations, any recommendations other than FPSs and RTSs?

Also can anyone tell me how I find out what my specs are, and what to actually post? I have absolutely no knowledge on this stuff.

So far, the ones that really caught my eye is what

non-gravity said (Rise of Nations and Empire Earth),

Dolby Digital (AQUARIA),

Slimebeast (Stronghold Legends),

salaminzer (Septerra Core: Legacy of the Creator),

and DaveD (Chip's Challenge)

Looking for:

Unique games
More WRPGs, JRPGs, and other unique games.

In front of my computer it says on a sticker:

This is the computer I'm using:

http://reviews.cnet.com/desktops/emachines-t6524/4505-3118_7-31556305.html
http://reviews.cnet.com/desktops/emachines-t6524/1707-3118_7-31556305.html?tag=mncol;psum

That's strange, I would have thought u'd have a much stronger graphics card in your PC.

That PC listed there shouldnt be able to run Rome or Medieval: Total War, not even close.

Your computer isn't THAT old....

The only thing that REALLY sucks is your GPU (on-board I assume). I never saw that card on a normal computer though...

So unfortunately no Assassin's Creed for you

you win, sir

I never said old, just crappy.

Warcraft 2 holds up kinda well today. With those PC specs, that's the kinda thing you might be limited to

I played both games totally fine O.o, although I was pissed off I couldn't play the European Barborum expansion for RTW, due to freezing and slowing down constantly.