I have a mobo with 2GB RAM (1GB X2). The exact specs on the memory on the computer is:

2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model 3AXT6400C5-2048K.

The mobo has two slots available.

Now I got 2 sticks of 512MB each that came out of another computer. I am reading the label on the memory and it says the following:

512MB 1Rx8 PC2 - 4200U - 444 - 12 - D1

Here are my questions:

1) Is this memory compatible with my mobo?
2) Can I mix and match with what is on there already (to make it a total of 3gb)
4) since the speed on the existing memory is 6400, is putting in 4200 really beneficial because I know that it will use the lowest speed.

Thank you!