I have been thinking about adding more memory to my PC for some time. I have done a scan to check what is suitable, and find I have just 512MB for my XP Home system, with 3 empty slots. Suggested upgrades are 512MB in two of those slots to bring me up to 1.5GB, or 1GB in all three empty slots, to bring me up to 3.5GB. That all seems clear enough, but then I am told I should use matched pairs. My current 512MB doesn't look like a matched pair, as it occupies only one slot. The proposed two 512MBs I suppose would be a matched pair, but the three 1GBs don't look like pairs at all, let alone matched ones!. Can someone please explain to me what I need to do?

Just go to the following link http://www.crucial.com/store/drammemory.aspx  They indicate you don't need matches.

Many systems require matched sticks. Even if your system does not one suggestion is to buy the best memory that your system can use even if it cannot exploit its full capability. This way you can hopefully reuse this memory if you upgrade your system in the near future. Even if you choose memory thats ideal for your existing system keep in mind that memory problems are difficult to identify and better quality memory is always a good idea.

Thank you for suggestions and sorry to have been a long time coming back to you. I tried the link from golferjack and ran the diagnostic, which definitely tells me "Your system requires that you install memory in pairs." I agree with Snakeyes about using good quality memory. I didn't get much help from online retailers, so I'll explore some local shops and find out what they suggest. If I have any joy I'll let you know!

Try Newegg.com. They are a very reliable and well priced online store.