Your CPU seems decently fast and should be okay for all but the largest maps with the most players. Like everyone you'll get a little bit of slowdown in the late game and on bigger maps, though - but that's to be expected.
Could i buy 2GB of RAM and add it to my current 2GB, or would i have to buy 4GB and switch them out?
Short answer: You should be able to do either, but it depends on whether you have two 1GB sticks of RAM, or a single 2GB stick.
Long answer: Your motherboard will have a certain number of slots for RAM in it. In addition, each stick of RAM will be a certain amount (1GB, 2GB, or rarely 4GB). So for instance if you have two slots and each one has a 2GB stick of RAM, then you have 4GB of RAM in your system.
I don't know how many slots for RAM there is on your motherboard, but my guess is there is two slots each with 1GB of RAM in them.
When replacing RAM you will need to make sure the type of RAM (
DDR2 or
DDR3) is the type that your motherboard supports or the RAM won't fit / won't work.
There is one last thing you should know: 32-bit operating systems are a little limited with how much RAM they can
read - which is around 3.5GB. What this means is that even if you have 4, 5, or 6GB of RAM; 32-bit windows will only be able to use 3.5GB of that RAM. This means to get the most out of your 32-bt system you need to get 4GB of RAM, and that it's a waste to get more than that.
While you're opening up the inside of your computer (you may need a screwdriver to get the side off) you may want to
gently clean any dust off the computer parts / fans.
http://www.youtube.com/watch?v=SiFIgSQOY7g - there's a tutorial on how to add RAM.
Also, am I right in assuming that since Civ 5 has some of the highest requirements of any game out there that this system will be able to run just about everything else, too?
Your system is pretty decent in general and will be helped a lot by getting the extra RAM. The reccopmmended requirements might
say that your video card is really good, but in reality you could probably stand to upgrade it eventually. But i'm not an expert on Radeon cards by any means so don't take my word for this.