To day it could be difficult to see what kind of computer you want. The market is great and the offers are many!
"Do I want a big harddisk, how powerful CPU will I need, is it an office computer?"
Question like these can go on and on forever! Therefor, it's important to decide what you are going to use it for!
Is it a gamer pc? For homework? Just a regular homecomputer? This is the kind of thing you'll need to figure out first.
A GAMER PC:
If you're of the "Hardcore Gamer" type and want a computer for gaming, these are the thin you should be looking for:
Make sure that you get a powerfull CPU (Processor) that can handle the system great aswell as the game it self. A CPU over 2,4 Ghz ir secomended for gamers. The next thing you want to see after is the graphic-card! This could be the most important part for gamers. When buying/building a computer for gaming, make sure you get more than 512 Mb. This is only recommend because the gams are only getting bigger and bigger aswell as the system requirements. Let's take the game "Crysis" as an example. This was the biggest game the world possible could have seen when it got prelansed. It had HUGE system req and very few had powerfull computers enough to play it.
The next thig you want to look after in a gaming pc is the RAM or memory. For a gaming pc it should be 3 Gb or more since games these days have 2Gb or more as req.
For Office/homework/desktop use
The office computers are as often used to write, calculate etc. By this kind of use the system you needed to be so powerfull.
Therefor, I recommend for the CPU to be something arround 2 Ghz.
Since you are not playing on this kinds of machine, 128 Mb is enough.
To speak of the memory, I don't think you will need more then 2 Gb. This is just to makee the whole system stable.
This rig is also for students. Also a big harddrive is important for every office users.
The regular "Home user"
What's this?
When I'm speaking of home users I'm speaking of people who just want a computer for the daily life! People who just pay there bills through it, perhaps have som kids who play little games, watch a movie or two...stuffs like that.
As for this you can combinate the "office-computer" and the "gamer-pc". You may like a good enough CPU on 2,2 Ghz, 256 Mb graphic and perhaps 3Gb of RAM.
IMPORTANT:
When buying a computer, it's most likely that you get a 32-bit system. This mean that the maximum gigs of RAM you can add/have on your computer is 4Gb. If building/buying a computer with 64-bit system and a 64-bit supported mainboard, you can go as far as 12 GB! Most of the mainboards only support up to 8Gb, but someone can go to 128GB wierd, but that's just how the OS is made