Like the majority of people, I say the same thing.
It all depends on what your needs are, how you use your computer ie: Business, Home, and an assortment of other variables.

First of all you need to weigh up what it is you want out of a computer.

. Entertainment - so you would be looking for a computer that has high quality sound and video capabilities pref. PCI sound and graphics card not onboard, whether you want to spend the extra money and have a Home Theatre PC which allows you to record from the TV to your HDD.

OR

. Business - Fast processor speed pref. dual or quadcore which basically means the processor can multitask between more than one application with out it slowing down or crashing which happens sometimes when programs such as Adobe Photoshop etc use cashing to create temp files.

You will find most people either tend to buy the high end PC out of lack of knowledge and get suckered in to the sellers cheap marketing tricks or just because they want to most out of their money, even though they are spending something like £600+ for a computer package.
If you were a first time buyer I would advice you to go ahead and choose a computer that most suites you from a computer store and then I would advice you to get yourself familiar with the computer. How to connect/disconnect components from the motherboard, what each component does and how to set the up also the most important know how to install and format your operating systems harddrive.

After you know the basic steps to setting up the computer, you then can start building your own. This is the best option for everyone as you will find out 75% of the time when you buy all the components seperately off the internet from websites like Dabs or Microdirect it will come out cheaper than a retail priced package.

Operating Systems

Using an OS at the end of the day is personal choice, some people will go for MacOSX, Linux, XP or Vista but one way or another you will bound to do all the same things a normal pc should, ie: opening programs, writing documents, running games, listening to music..

At the moment I have two computers, Acer Aspire 5101 laptop which is running Vista which is a more "entertainment" based computer and my desktop which is more "business" orientated, which I dualboot XP Pro SP2 and Linux SUSE-XGL on. I'm happy with Vista at the moment, people might say some things on Vista get annoying ie the added security which has a popup alert which makes you accept every major action stated on the machine, but I never use that feature as I am not really a novice user so I have no need to be protected against threats, as I make sure I never get any.

The one thing I advice you is never listen to just one person do as much research as possible before making a choice. You will find most people who answer this question are bias so it is hard to really gain any advice from people who are just explaining it from their point of view but not the broader aspect of it.

Monitor
With more and more websites being designed for widescreen high res displays and games being more and more detailed you should switch to LCD

CRT monitors (generic boxtype) are OK if you are just using your computer for a few hours or so but if you are doing design work, office work and on the computer for more than 1-3 hours an LCD is the way to go especially if you intend to use it for home theatre system.

I have just recently bough a 19" Viewsonic VG1390 widscreen LCD monitor which I use as dual monitor between my laptop, widescreen monitors are very good if you tend to open up alot of windows or programs as you get slightly more on the screen.