According to this web site ( http://www.memorystock.com/memory/CompaqPresarioSR1522X... ), you can use 4x1 GB sticks of PC2-4200 RAM. As long as frequencies and, especially, voltages are the same, you can use one stick of 1 GB and one stick of 256 MB together. If voltages of different RAM sticks are not the same, you may have a problem. The higher voltage at which one stick will work may damage the other stick that tolerates a lower voltage. If frequencies ( speeds ) are different, your computer will operate at the lower speed of your RAM stick.
If it turns out that you can run different RAM sticks, it is wise to install the larger RAM stick in the lower numbered slot