Terry - I don't know if that link above helps you any - it isn't the correct board but one with a similar chipset (6150 series) As you are already aware you can only put 1GB in each of the 4 slots as it appears your board can only support a total of 4GB. Shop around for RAM prices - your best bet would probably be to go for 4 x 667
Run SIW to obtain the correct details - ie: memory type, memory speed, supported speed, memory timings, correct voltage in addition to what the Crucial scanner tells you.
Terry - I am assuming your RAM is PC2-5300 667MHz - older RAM doesn't come any cheap these days - there are 1GB modules here enter link description here or another option would be E-bay but having had a quick look on there it doesn't appear to be any cheaper. If you would like me to look around for you I am willing to do so.
Terry - I agree with Chronus with regards to noticeable difference - but if you decide to upgrade then go for 4 x 1GB (all 667 or 800) If you go for 667 then you will just have to purchase 3. At the moment, as you rightly say your working speed is 533MHz