yes you can use it with it, and it “can” make it slower.
Your memory will only run at the slowest speed of all the sticks, which is why it’s critical to make sure it’s ddr400/pc3200 which is the optimal speed of memory for your standard amd 64.
the other issue is dual channel. For all intel P4’s, and some amd 64’s the memory can run in dual channel which effectively doubles the performance (in theory). To run in dual channel you must have either 2, or 4 sticks of memory and they must be placed in the right slots.
So if you go from 2 to 3 sticks of ram, and knock yourself out of dual channel you might have more ram for large tasks, but take a severe speed/performance hit.
Dual channel is more of an intel thing anyways, amd doesn’t see as much of a performance boost from it.
The other issue would be overclocking. For large over clocks you need very good ram, but if you are asking what to buy you probably won’t be doing that anyways.
As for your motherboard there is no way to know how good it is unless you know what model it is. But for your needs at the moment all you really need to know is if it’s AGP based, or PCIE based.