I have a couple of thoughts on this. If you REALLY want to know what these terms are, start reading. Read as much as you can. Post in a new thread any questions you might have. This is really the only way to learn. RC did a good job, but there is so much more to bit width then he covered, nor would I expect him to hit it all. (Frequency, and type of DDR also play a huge part in memory through put.) You have to take everything into account, or you might not make the right choice.
The second way is to do what you did. Post in a forum and ask what the best card you can get for $$$. Notice that I said it was the 4850, along with some others. This is your hint that this is the best card you can afford. Who cares what the specs are if its the best you can get? Does it make any difference if it has X Vram instead of Y? Or that the bus is X bits wide instead of Y?
If you really want to know, I'd start with the wiki pages related to video cards. That should be a good primer. After that, start searching for articles from toms, anandtech, xbit labs, etc for more detailed information. Don't forget to also read the stickies in the video card forum on this site as information is there as well.