First off welcome to the forum pal 8)
and why do u think that there isnt any substantial increase in ur pc's workin pattern
bascially after an additional ram being added the apps being to load faster,games load time decreases,boot time too decreases(to some extent),faster installations of applications,faster copying of files hard disk and Cd media's etc.....if none of these are happeing in ur pc then there might be a possiblity of a fault.....the best way to find out would be to test a games loading time.....1st try with just the 256MB chip on....notice the time taken....later add the 2nd chip and then note its time taken...compare the diff and u can make it out
also try reducing the tasks running in the background cause they take up some amount of ram