hi, welcome to hardware forums. :D
Your question is not a stupid question, someone has to start from somewere, 2 years ago i never knew anythign about computers, infact, i was asking questions like yours.
sound cards fit into a PCI slot on your motherboard, a pci slot is usually a cream connector on your motherboard, and most motherboards have 3 pci slots or over, but some recent motherboard onlt have 1 or 2.
sound cards are for delivering sound, they are much better than built in sound to your computer as built in sound cards usualy get interfearence from other components, sound cards also have better quality sound eg 24-bit
graphics cards fit into an AGP or PCI-E slot, depending what motherboard you have, it may have a PCI-E or AGP slot, PCI-E is not the same as PCI, they are tottaly different, graphics cards are obviously for graphics, if you have a game like doom 3, and want to play that at maximum settings, your going ot need a very good graphics card, which will cost you lots of money, or maybe get somthing like an X800GTO2 which is quite cheap, and modify it to an X850XT PE graphics card, which is v high end, same goes for graphics cards here, they have much higher quality than intergrated graphics cards, as they dont have there own memory, they share the ram memory, also with graphics card, memory size does and does not matter, you could have a 128mb card that is better than a 256mb card, but with high end cards, they dont go under 256mb, its usually 256 or 512mb.
before you buy a graphics card, check what slot you have, AGP or PCI-E, use a program like
everest to find out what type of slots oyu have, it may be so that you have a really old motherboard and it only has pci connectors on, so you might consider a cpu and motherboard upgrade there too, as pci graphics cards are not good for modern use.
Everest will also tell you everything else about your computer, you may want to try CPU-Z, this is a very small program that opens straight up from an .EXE, and no installation required, good for information on ram and your processor.
I hope that helps you :)