How hard is it to install new video card?

Thinking about upgrading my computer. Two things. Better graphics card and more ram. Would like to do the work myself. Dont know a thing about dos screens or building a computer. Can a techno numbskull like me pull it off?

I know my mother board (Gigabyte N-Force 4) supports nVidia as I currently have a GeForce 6200TC. Do I have to stay with that technology? Is there a good top of the line nVidia card? A lot of people praise the ATI Radeon card, is it the 9600 or 9800. Would an ATI card work with my mb?

How bout ram? Is it basically "plug and play" or do I have to punch in a bunch of numbers on some dos screen. Is there a compatability issue with what I currently have?

I know all these questions might sound silly to some of you but I just want to learn how.

Any suggestions most welcome.

PS
by the way since this my fourth post it's high time that I should introduce myself. Real name's Richard. Have loved airplanes since as long as I can remember. Also spend 30 years jumping out of them. It's so cool to go to all the airports that I've jumped at. Anyway I digress.

Cheers