It is possible to have to much from what I've heard. It all depends on how much of you memory that your chipset can cache at one time. The VX series chipsets from Intel were only able to cache the first 64 MB of RAM, so adding more than that could actually slow your PC down. While HX and after were able to cache the entire first 512(?) MB of memory. I am not sure how the newer chipsets are, but I feel confident that they will handle anything up to 512 MB. If I am wrong on any of this, someone please correct me.
All the Intel Socket 7 chipsets (FX, VX, HX, and TX) had the max 64 MB cacheable limitation.
However, the HX had an option to support 512 MB. It has to be enabled in BIOS and an extra tag memory chip must be present on the MB. Problem is that most MB didn't come with this chip and some don't even provide any upgrade socket for this chip, effectively preventing it from caching more than 64 MB.
In some cases, a system with 64 MB RAM can actually perform better than with 80 MB or 96 MB.
Newer chipsets for super socket 7 and PII don't have this limitation. I'm not sure how much the limit is, but I'm sure it's more than 512 MB. For these newer systems, get as much RAM as your wallet can afford. =)
Mole is correct with his staments. i would just like to add that newer chipsets are capable of caching up to 512m, but your L2 cache dictates if you can caches that much. the newer chip sets allow them to recognize and use the increaded amount of RAM, but you have to have enough L2 cache to allow for the increase in RAM. i am not sure how this works with PII MoBo's. with socketed boards 512k cache can effectively use 128m of RAM, 1m L2 cache can effectively use 256m RAM, and i believe you need 2m L2 cache to use 512m of RAM. you can always add more RAM, but it can slow the system down when trying to access data stored in RAM that is not cached. having to go to RAM and not the faster L2. i am not sure about the performance hit. i would be curious to know those numbers.