i concur that a prime is the way to go for concert shots. zooms with a large maximum aperture are expensive. i have a 50mm/f1.4 prime and a 24-70/f2.8 zoom. i can get good shots with the 2.8, but the 1.4 excels in low-light. depending on where you shoot from, the 50mm prime is most likely your best/most affordable bet. it won't be great from long distances, but fast telephoto primes get expensive as well. from a quick search on the B&H website, i found 2 Minolta 50mm primes: the f1.4 autofocus for $239 and the f1.7 for $77. i don't know anything about the relative image quality of these two lenses, but the 1.7 for $77 sounds like a very good deal. you will be able to shoot in concert environments with a 1.7 max aperture. the 1.4 gives you an extra half-stop and if minolta's line is similar to canon's, the 1.4 is likely built a little better and with slightly better autofocus. but, as MattD said, you may find yourself having to manually focus in extreme low-light situations anyway. if you don't want to spend a ton of cash, the 1.7 sounds like a good buy. but i would check reviews at photographyreview.com before i bought.
EDIT: B&H has that 50mm/f1.4 used for $159. sounds like a good deal. i don't know which minolta lenses fit your camera so i would double check that.