the difference is not as great as when moving from a regular tv up to a hd ready , 720p tv , though even then , the difference wont be appreciated unless the content is hd ...
also screen size is a factor , if your moving from a 37in hd ready tv , to a 37in full hd tv , you wont see much improvement , but if your talking about 50in tvs , a full hd one would be a bit sharper , and show a bit more detail , than hd ready at that size ....
all of the above is only true when viewing a good hd source , sky hd , blu-ray etc ....