if you are using irons, almost any scope would be an upgrade. i have an old marlin model 60 that i got from a friend, the rear sight was broken so i got a cheap 40mm walmart scope for around $20 and gave it a good cleaning. it shoots fantastic, long range head shots galore... it has replaced my 12 gauge as my "go to" squirrel gun. and it's WAY more fun...
to me, the ammo makes the difference. a hand full of cci mini-mags and i'm in heaven.
if it's a nice gun and you have money to spare then maybe go for high(er) end. but if you just want a play thing for plinking or actual field use, save your money toward that new bow you've been wanting. that's what i did, and i couldn't possibly be happier about it. and as a bonus, i LOVE my new bow... took it out today as a matter of fact...