Hello,
Got your message after you posted your question.
From my research, the Sempron CPUs have a 333MHz Front Side Bus.
So, any of the DDR modules you are looking at will function properly.
The DDR 333 is all that you need for the CPU.
The DDR 400 PC2700 is an anomaly. It reads like it supports a 400 MHz FSB, but only runs at PC2700 speed.
The DDR 400 PC 3200 is faster than you need now.
Unless it is cheaper, or you expect to upgrade to a CPU with a 400 MHz FSB, I would advise against buying faster RAM than you need.
Check and see what RAM speeds your motherboard supports. Some users have had problems using faster DDR RAM than is supported by their motherboards.
Chas