Too sum it up quickly, AMD is for gaming while Intel is for "home/office" use.
I belive there is a sticky on the forum more about this subject that explains it in detail.
The burning discs issue, is it CD or DVD you want to burn? Or both?
The 48x was most likely for CD burning while 16x is pretty standard for burning DVD. If you get any standard DVD burner you will also be able to read and burn regular CD's as you wish.
Most likely it was just a regular CD burner and for an additional $30 you could have it upgraded to a DVD burner.
$55 for an additional 512 mb of ram depends all on what you plan to use the machine for. You did not specify how much memory you had in the machine from start so I will assume you have 512 already.
If you just plan to surf the web and do minor tasks on the computer 512 is enough, however an upgrade to 1GB for games or the alike is always a good choice, if nothing else you'll be a little prepared for the future
$55 is a rather standard price, you can go lower or higher depending on quality and effiency of the memory but I would say its pretty standard.
If you want more info please specify a bit more what type of use the computer will be for etc. The more information you provide the more help you will be able to get.