This advice is for you if:
- You're a mainstream computer user who will use their computer like 75% of people out there (office documents, photos, Internet, music, email)
- You don't care or probably won't ever upgrade your machine, open it up, understand what a SATA cable is, etc, etc.
- You don't need to be able to play the absolute newest computer games as fast as they can possibly be run
- You're not going to be doing graphic or video editing or other resource intense things on your computer
- You don't have a need to run 12 or 24 programs all at the same time on your machine
Ok on to the advice:
What Brand - Buy a well known brand: Dell, HP, Compaq (made by HP), IBM/Lenovo, Apple etc.
New or Used - Computers are cheap, unless you're really strapped for cash and willing to take a gamble, buy new.
Build Your Own or Buy - If it's a hobby and you have the resources (time, knowledge, and money) to spend on it feel free to build your own system from buying all the individual parts. It's not rocket science but it will take some time and you do risk having problems along the way. You may save a little cash but if you factor in your time it's probably more expensive to build. Plus in buying you have one company to deal with if it breaks and one company that will ensure that all the components in the machine all work together.
Warranty - Get an appropriate warranty. Know what warranty you are buying! Some require you to ship the machine in which you may or may not like. There is also extra 'accidental' coverage that applies if you spill a drink on your laptop. Maybe you need it, maybe you don't. Match warranty to need and cost. Be emotionally ok with the machine breaking 1 day after your warranty is expired.
Cost - If you don't need it yesterday hunt for a deal. Check paper ads for your local electronics retailer, Dell Specials, slickdeals.net, Black Friday/Cyber Monday, that kinda stuff. Don't spend a lot. For the cost of 1 top-end machine you can buy a middle of the road machine and completely replace it with an even faster machine in 2 or 3 years.
Buy Online or From a Store - Both have their pros and cons just be aware that there will be variance on the knowledge of sales people (just like with people on the Internet) and take everything they say with a grain of salt, like any sales person :) If you buy online, be sure to buy through a reputable retailer.
PC vs Mac - There's quite a bit of info/opinion out there but my 2 cents: Macs are nice esthetically and functionally and generally more polished (though PCs have gained on them in recent years if you ask me). While Macs seem to have less problems in general, they are more expensive, less flexible, and have less software options than PCs. One thing noteworthy about Macs is that if you're near a Mac store you can take your machine in to the 'genius bar' and have a real person help you with it if you're having problems. I haven't used this service myself but would bet that this is superior in most cases to calling someone in India or whatever other foreign country. Some of us are Macs, and some of us are PCs. The choice is yours.
32 vs 64 bit - Windows comes now in 32 or 64 bit editions. You only need 64 bit windows (x64) if you need more than 3 GB of memory. However having 64 bit windows on a system with less than 3 GB of memory isn't going to hurt anything either. Most programs and things today work on either but feel free to check if you have any things that HAVE to work on the new system.
Other Stuff - If you have a printer, scanner, camera, mp3 player, or anything else that you want to connect to your machine check that it will work with your new version of windows or mac!
Store Services - Don't buy add on crap or warranty from a store (aka Best Buy) unless you are really dumb or lazy or rich. Again, unless you're one of the previous adjectives, get your warranty branded from manufacturer. Realize that retail stores will lie and pressure you to purchase things you don't really need. Its one of the ways they make money. Half of what they do is probably explained out on the web. Google is your friend.
Memory - Memory upgrades are sometimes cheaper to buy separately instead of an upgrade when you're building/buying your system. www.crucial.com makes it quite easy to price memory upgrades and installation is usually about as hard as a screw or two. Most systems today run great on 2-3 GB of memory.
Programs - If you need office or other programs its usually best to buy it with the machine if available. ALWAYS run anti-virus protection. There are decent free ones out there at the very least. Don't forget to back up your important data! Computers and drives do fail! If you don't want to deal with having backup drives around www.Mozy.com is a good online backup service.
Size - For both desktops and laptops think about physical size of the machine. There is a lot of variance out there. Smaller desktop computers are nice however are not as easy to get into or upgrade. Laptops as well range in size from 9-inch up to 17-inch screens with everything in-between. Retail stores are great for getting a feel for the size of the machine and/or monitor that you will want.
Hopefully this post is useful to you if you're looking at buying a new computer. While it may not cover every single detail it should cover most of the points to consider in your decision. Happy computing.