Max Ram Dell motherboard can take

I have a Dell pc, the motherboard is OWj 772 serial no CN7082167R0030 and would like to increase the
Ram ,it is a 64 bit system running windows 7 ultimate.The present ram is 1.5gbs consisting of 1 gbs and a separate 512gbs slot .I have read that it may or may not take 4gbs,how do I find out? If it doesn't take properly what would be the signs i.e not boot up etc, in short when I fit the new ram assuming it can be supported by the motherboard how will I know it is not sitting idly there not improving the situation.I read somewhere on this site I believe that Dell support told someone their system could only support a 2gbd ram when in fact they were wrong and it supported 6 gbs.I don't know where to start fitting new Ram but am assured it isn't too difficult