Can someone explain why adding more memory to my computer would slow down my games?

Couple Possibilities 1.If you have more than 512mb Memory performance may be decreased because Windows 9x/Me only supports up to 512mb. 2.More DDR RAM uses More Volt. PSU Watt may be too low, you may need to get a better one. 3.Lets say if your current RAM is Pc3200. If you add some RAM that are Pc2100, then your Pc3200 RAM will run at Pc2100 speed. (Note : Pc3200 is faster than Pc2100) 4.BIOS needs to be updated. Aspire X-Dreamer II Thermaltake 480W Silent Purepower PSU Asus P4S800 MoBo Pentium 4 2.6ghz @ 3.0ghz eVGA Geforce FX 5900 SE Creative SB Audigy LS

Another option is just plain old incompatability between the two memory chips. I don't suffer from insanity, I embrace it.

Too much memory can slow a computer down. The limit depends on a lot of things, but primarily on how much and what kind of cache’ is installed. It does indeed take system resources to manage memory. If you exceed 128 Mbytes on many computers with old chipsets and smaller cache’, you will slow them down considerably because they won’t use the cache’ at all.