What you are calling "chips" are the DIMMs, the memory modules. The chips themselves are the black things on them. That's relevant for your question. what's also relevant is exactly which motherboard you have - or rather which motherboard chipset it has.
Basically with a P3-750 you have three possibilities:
- motherboard chipset supports max 128Mb chips (note lower case b) without artificial restrictions (i.e. i440BX)
- motherboard chipset supports max 256Mb chips without artificial restrictions (i.e. Via ApolloPro133A)
- motherboard chipset supports max 256Mb chips, but is limited to 512MB by design (i.e. i815)
Now, a regular unbuffered DIMM can contain max 16 chips. So if your motherboard supports max 128Mb, your max DIMM size is 128x16/8=256MB with 16 chips. However, the max size for an 8-chip DIMM is 128MB. With max 256Mb chips, your max DIMM sizes are doubled, so 512MB with 16 chips and 256MB with 8 chips.
So, if you have an i440BX-based board, you can use 256MB DIMMs if they have 16 chips each, or 128MB DIMMs with 8 chips. With 3 DIMM slots, you can install max 768MB. If you have a Via ApolloPro133A, you can use 512MB DIMMs and 8-chip 256MB DIMMs too, for a max of 1.5GB. However I concur with earlier posters: Win98SE starts misbehaving over 512MB RAM- there are workarounds, but there is no sensible reason to want to run so much RAM on an OS this old. I'd recommend sticking to 256MB or max 512MB.
Speed ratings are like road speed limits: a DIMM is run at a certain speed by the motherboard. Running it faster than its rated speed risks crashes. Running it slower is perfectly safe. Your P3-750 has a 100MHz FSB, so if you run the memory synchronously, PC100 and PC133 are both fine. If you have a motherboard capable of asynchronous operation, you can run the memory at 133MHz even though the FSB is only 100MHz. If you want to do that, you need PC133 to be safe.