What's your end goal here? An extra speed or two on a MTB really isn't much to go after IMO.
+1 that it would cost more than the bike is worth to upgrade to 8 speed, and even more-so if you want 9-speeds. For 8 speeds you'll need a new right shifter, 8S cassette, and a new rear wheel with a Freehub. If you want 9 speeds you need all of the above plus a new chain, and possibly a new crankset and front derailer if the front doesn't play well with the narrower chain.