When you buy memory to upgrade a system, consider the following issues:
Brand name is important for memory. In our 20+ years of working on PCs, we recall only one high-quality name-brand module that failed other than from lightning damage or other abuse. Conversely, generic memory fails quite frequently. We recall one batch of 20 cheap DIMMs ordered by a client in which fully half of them were dead on arrival and the others failed within a few months. For general-purpose memory, we exclusively use and recommend memory modules from Crucial Technology (http://www.crucial.com) and Kingston Technologies (http://www.kingston.com).
CHEAP AT HALF THE PRICE
Some companies including Corsair, KingMax, Mushkin, OCZ, Patriot, and Viking sell high-performance memory for gamers, overclockers, and other enthusiasts. Crucial also competes in the high-performance segment with its Ballistix modules and Kingston with its HyperX modules. Unless you are trying to eke out the last percent or two of potential system performance, it's seldom worth paying the higher price for these premium modules.
It's generally less expensive to buy mid-capacity modules rather than the equivalent amount of memory in large- or small-capacity modules. For example, if you need 1 GB, two 512 MB DIMMs will probably cost less than four 256 MB DIMMs. Conversely, one 1 GB DIMM may cost 50% more than two 512 MB DIMMs, and a 2 GB DIMM may cost twice as much as four 512 MB DIMMs.
It's tempting to leave the old memory installed when you are upgrading a system, but it's often a mistake. For example, if you have purchased two 256 MB PC3200 DIMMs for a system that currently has two 64 MB PC1600 SDRAM DIMMs installed, you may be tempted to leave the old DIMMs in place, for a total of 640 MB rather than only 512 MB. After all, you paid good money for those 64 MB DIMMs. The problem is, leaving the old DIMMs installed will cause your new PC3200 DIMMs to run as PC1600 memory, crippling system performance. In general, the best course is to remove old memory rather than retain it.
Older types of memory may be very expensive per megabyte. For the same cost as adding more old-style memory, you may be able to purchase a new motherboard, processor, and the same amount of newerstyle memory. In that case, opt for the wholesale upgrade, which effectively gives you an entirely new system.