Well, I'm currently upgrading my PC (its a good 6 years old, but still runs most things fine [But will need an update for upcoming titles {BF3}]), and I am not replacing anything but my Motherboard, RAM, and CPU (Maybe Graphics, my NVidia 9800 GTX+ is getting a bit old now).
Basically, if your computer is old, your going to need to upgrade RAM, CPU and Motherboard, for compatibility reasons. Now, I'm not the best with computers, but after 10-15 minutes of reading on a good site, I learned to link a few things together.
Now, for CPUs, you have two brands: Intel and AMD
Intel are more costly, however, IMO, more reliable as well (I have one of each's CPU in separate computers, and the Intel one tends to run better. AMD are a fair bit cheaper however, and there isn't a ton of difference in how they fare for my PCs, so go with what you'd prefer.
RAM, all you really got to worry about here is having the RAM compatible with the motherboard and CPU, if its not, you'll have issues with all sorts of things. Usually, the DDR# and the 4 digit # (Mine is 1333, not sure what this is) for compatibility, but there may be other things you should look at too (# of pins perhaps?)
Motherboards are basically there to be compatible with everything else and be the nexus of the computer. There are a ton of numbers for these things, so I just look for things that are the same as my CPU and RAM in the numbers I think are important there.
The components I'm thinking of purchasing are:
http://www.msy.com.au/product.jsp?productId=7100
http://www.msy.com.au/product.jsp?productId=6870
http://www.msy.com.au/product.jsp?productId=7548
and should cost me under $800. I believe they are compatible (If someone more educated in this issue could check, it would be appreciated), and the site has most of the specs (I think) for each. Its a local seller in Aus though, so unless you live here, I'd find another site selling similar items (NewEgg perhaps?)
