The computer will run at the speed of the slowest module. You are looking at 400 MHz in the PC3200, while the other one is PC2100 266 MHz.
You won't notice much difference in actual use, but it won't do you a lot of good either. Performance will be helped hardly at all. It is the memory capacity that will make the difference. And you do not have enough memory for most tasks if you are going online. Once upon a time 256 mb was adequate, but with Microsoft updates and the automatic updates of other software, you are probably getting hesitation or freezes.
You can buy a second module of 512 mb PC3200 for $25 at zipzoomfly.com with free shipping.