<A HREF="http://hpshopping.speedera.net/www.shopping.hp.com/shopping/data_sheet/dc475a.html" target="_new">This link doesn't give me much to go on.</A>
Okay... you have two DIMMs.
You probably have a crappy chipset with a limited memory controller. Either way I would stay within Compaq's guidelines of 1024MB.
With that being said, you can buy one stick of 512MB of PC2100 or two and ditch your existing stick or buy one and insert that existing stick.
So you will have 512MB, 1024MB, or 768MB based on which option you choose.
You can buy some Crucial if you like or someone elses. <A HREF="http://www.crucial.com/store/listparts.asp?Mfr+Productline=Compaq+Presario.&mfr=Compaq&cat=RAM&model=Presario+S3000NX+Series.&submit=Go" target="_new">Here is a link to Crucial if you don't want to look around for yourself.</A>