When buying a new computer all you need to consider is the following: What are you going to use it for? Do you want a flat screen (LCD) monitor or an older style monitor (CRT)? What level of support do you want?
It may seem too simple but believe me, there isn't much else to it. Think about it, Computers have been commoditized. Some people may disagree but for normal, everyday people they don't care who made the thing, they're all the same. As long as it works when they press the power button then who cares. That is why you only need to answer the 3 questions above to determine the type of computer you're going to buy.
Question 1: What are you going to use it for?
I split this into two categories: Computers for games (real games, not solitaire) and Computers for email. If you're buying a computer and need/want to play the latest games on it then you're going to have to spend more money, period. You need a faster processor, more memory (RAM), and a good graphics card (ATI, NVIDIA). You should be paying at the higher end if you're getting a good gaming system.
Then there is the computer for email. This is the computer where you check email, write reports in MS Office, and chat on MSN messenger. You don't need much power for this and even most of the low-end systems you buy today are overkill for these types of tasks. The only suggestion here is that you get at least 512MB of RAM.
Question 2: What type of monitor?
For gaming I suggest CRT monitors (the older style). They still produce the best image and have a faster drawing rate. LCD is easier on the eyes and takes up less space and I think is preferable to everyday normal people.
Question 3: What level of support?
As I mentioned, computers themselves have been commoditized but service is where providers can differentiate themselves. A lot of people have that guy they know who fixes their computer when something goes wrong, usually a brother, or friend. If you don't know anything about fixing computers or don't know anybody then you might want to opt for a company that is reputable in these matters. Sticking with larger retail chains or a company like Dell would be your best bet.
If you have recently purchased a computer or can offer any advice please leave a comment in the discussion area.