The AT FAQs have an
article about how to tell chip density. However it depends on you being able to get certain information about the chips used on a module, which isn't always available, especially when you buy online and don't know the exact chips used.
The density is dependent on the organization of a single chip on the module. Older chipsets couldn't support chips bigger than 64megabits, and the next generation went to 128Mb. Current chipsets support 256Mbit and higher densities. Eight chips at 64Mb each makes a 64MB module, and 16 chips makes a 128MB module, which is why older boards say each slot can support 128MB modules; some modules also stacked chips to allow double the size without using higher density chips.
The speed issue is separate from the density issue. It just happens that higher density chips come out while slower speeds become obsolete; nobody is making PC66 memory or chipsets anymore, so nobody is making a PC66 chipset that can use high-density memory, and memory makers don't want to make low-density memory because it's less cost-efficient.
PC133 memory can physically run at PC66 speeds. However many modules don't have the SPD programming to automatically run at those speeds; they only have PC133 speed settings. Most OEM computers use a motherboard that doesn't allow you to set the memory speed manually, it will only detect the SPD settings. If the board can't find the PC66 rating in the SPD, it will assume an unsupported memory type is installed and fail to run.
If your board has manuall control over the memory settings, and supports the particular density of the chips, then you can use PC133 memory in a PC66 system.
The exact model of the motherboard you have, or the computer if it's a brand-name, would allow you to find out what it supports. However given that it is a Pentium series, it is unlikely to support anything higher than 64Mbit density chips, arranged as 4Mx16 or possibly 8Mx8 or 16Mx4. It may not even support that much.
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