It annoys me a little that sales people want you to spend anywhere from hundreds to thousands of dollars on a computer basically sight-unseen, or flood you with techno-babble that is as meaningless to them as it is to you.
Consider the computer sales staff much the same as you would a car sales staff. They are NOT your friends, their job is to sell you a PC, preferably an expensive one. In fact other than specific, pointed questions you should ignore them.
First tip: Get a laptop. They are sleek, sexy, portable and can do anything you need a computer to do. IF you are planning on installing 8 hard drives and liquid metal cooled overclocked CPU heat sinks, this is not the guide for you anyway. If possible get a good sized LCD monitor a keyboard that you like, and the pointer device of your choice. You'll need those for a desktop too. Don't get a netbook. If you need THIS guide, you probably won't have much use for a netbook. Cool as they are, they aren't a full computer and you'll be wanting some capability the netbook doesn't have sooner or later.
Try out the computer. Type on the keys, use the touchpad or mouse. See how it feels to you. Does it fit your hands? Can you read the screen?
(At this point the sales staff tends to start hanging about. If you engage them in conversation, they will immediately start asking stupid questions and trying to sell you something more expensive. It's not their fault, just doing their job. Ignore them.)
Now, make the computer work for you. Open and close some programs. Reboot. push the "special" keys if it has them. Is it slow? Does it respond crisply? Does a particular activity seem to cause it to stop or "freeze" even for a second or two? IF so, move on. This is a brand new computer. If it's draggy now, wait til you've got a few months worth of web surfing and forwarded email on it.
(By the way, at this point the sales staff will get nervous, either because they have no idea what you are doing, or because they suspect that you do. Either way, ignore them.)
Do this for more than one computer, even if you fell in love with the first one. Anything that seems wrong or off-putting is absolutely a good reason not to buy it.
Regarding the specifications, the techno-babble: For the moment processor speed is not a limiting factor. That is, your processor can handle more than you are likely to throw at it. You can pretty much assume that part is more or less on par. The two spec you _do_ need to note are memory and drive space. They are confusing because they are measured in the same units, "bytes". Memory is usually much smaller than drive space. A good memory level for summer of 2009 is about 4GB (four giga bytes, sometimes referred to as 4 gigs). Hard drive space is usually hundreds of GB. Maybe up to 512GB by now. Either way, more is better on both of these - but not to the point of a custom build. That is if someone offers to add memory or drive space, pass. It'll be cheaper to buy a slightly higher end computer.
(If at some point the store management has shown up asking what you are doing in a polite but firm manner, you should probably politely but firmly tell them that you are test driving their computers and plan to purchase one soon, you'll call them if you need them, thanks.)
Do NOT buy a computer yet.
Go to another store (or ten) if feasible and repeat the process. Make yourself wait at least 24 hours before buying the one you choose. If it still looks like a good price and still feels right, go for it.