I was reffering to specifically what type of memory we're talking about here. For example, say you currently have 2*512MB of PC2700 DDR1 and you want to add it with the other 2*512MB PC3200 DDR1 in an AMD64 system, you can technically do so although all memory will operate at DDR333 and you'll have memory bandwidth issues
If you are using DDR1 and want to switch to a Socket T Pentium 4 you probably won't be able to as most 775 cores will want DDR2, and even if you have the option of using either DDR1 or DDR2, you're going to again be creating a bottleneck especially with the Pentium since it relies heavily on the faster DDR2 FSB speed