Your motherboard might be fussy with what ram you use.

Have a look on the net for a Memory test program. You boot it up from floppy disk and leave it running a series of tests (over and over) it will highlight any failures.

If the rams not compatible with your board you'll find it keeps failing on the same tests.

If this is the case you'll have to fork out for some better ram, give these guys a try http://www.crucial.com/uk/

you search for your motherboard and it lists their ram which is 100% guranteed to work.