Alright, when it comes to computers I'm usually well versed, but when it comes to buying things that increase my computer speed or memory my brain turns off. I know where to buy shit like that, but how do I get them into my computer? Here are some of my biggest concerns, even though I assume these questions have probably popped up somewhere down the line in the existence of this section...

If I get a new video card, do I need to get a sound card as well?
If so... what's a decent video card, and sound card?
Do I have to put something into the harddrive to increase the GHz/Gigs or do I have to revamp it completely?
How much would 50 Gigs be? (I have 20 on this computer.)

Not that I expect to take care of all this at once, but it'd be nice to know all this for good. Thanks.