Building up an obsolete machine is something you do for the love it rather than because you need a more powerful machine. As you stated, more powerful machines are cheaper than building up an older one so nobody can really tell you what to do. If you want to do it and don't mind the expense, then do it. If you would rather use the money for a newer machine, then do that. Whatever you do, though, don't let that old Mac go to the recycling center before you offer it to someone here!