No we're mixing up two things hear, memory capacity and memory speed.
Capacity is in Meg and speed is in Mhz.
You can mix up different "Meg" memory sticks but not advisable if your mobo has dual channel capability.
As for the frequency or Mhz; PC2100 for instance has a frequency of 266Mhz. Now is you put in a memory stick PC3200 in ( 400 Mhz ), your mobo bus speed will only be as fast as your slowest memory stick, in other words in this example 266 Mhz. So putting a 400 Mhz stick will not improve your system speed. The 266 Mhz is creating a bottleneck. So put same frequency RAM in your mobo, and preferably of the same frequency as the mobo bus. No mentions about the cas latency of the RAM.
Asus P4P800 SE
Intel P4 3.2 Gig 1024 Meg PC3200 cas latency 2.5
RAID 0 2-36.7 Gig SATA WD 10,000 RPM
200 Gig WD ATI ALL-IN-WONDER 9600XT
6500 Kb bandwith Internet Connection