With today's economy going into the dumper, it seems clear that fewer computers will be moving off the shelves this fall and winter season.

That will mean more computer upgrades as users try to get more performance out of aging machines. So let's discuss the upgrades that make the most sense, and when to upgrade and when to go for a new machine.

First of all, if your desktop machine is more than six years old, it is likely not worth upgrading. A simple check is if the machine has a sticker on it indicating that the computer was designed for Windows XP or Vista, it likely is worth upgrading. If the computer is made for Windows 98 or before, I would consider it too old.

First, take a look at the amount of system RAM (memory) in the computer. Many computers more than a few years old came with 256 MB or 512 MB of RAM. I would consider a RAM upgrade to be the most cost-effective upgrade on the market today. The trick, of course, is to find out what kind of RAM your computer needs and how much your computer can hold.

That is a two-phased question. One is how much your computer can hold and how much is in there already, and the other is what kind your computer can take. Today, most computers take either DDR memory or DDR2 memory. DDR is about twice as expensive and is used in older models. DDR2 RAM is fantastically cheap; I have seen DDR2 RAM in 1GB size for less than $15.

A great site to determine what your needs are is Crucial's Web site. It has a memory calculator where you can type in your computer model number, and the site will tell you the capacity and type of RAM you need.

Installation of memory is simple, and there are many guides available on the Web. If you are performing this task on a desktop, get a can of compressed air to blow out the dust at the same time. Make sure you unplug the computer, too. On a laptop, make sure you unplug it and take out the battery before you install memory.

Another upgrade is storage space. Adding a second hard drive to a desktop is rather easy, assuming you don't have a second one. Just determine if your computer has IDE drives or SATA. You can tell by opening your computer and looking. IDE drives have the older grey ribbon cables. SATA drives have smaller data cables that are often red.

If you buy one in a retail box, you will get the cables and instructions needed. Pricing around 50 cents a gig is not unusual.

Upgrades to avoid would be processors (today it is rarely worth it for the consumer), video cards, and sound cards.

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James Derk is owner of CyberDads, a computer repair firm, and tech columnist for Scripps Howard News Service. His e-mail address is jim@cyberdads.com.