There will be a huge difference between an 80's mtb and one of today's versions. My mtb is about 15 years old, which would make it mid 90's,and I've been thinking of upgrading, The newer bikes are so much lighter and the components and disc brakes are so much slicker, that it would seem like a no brainer to move up if you are going to get more serious about riding. Also depends on whether you want a hardtail or full suspension. The most bang for the buck would be a hardtail, IMO. I am amazed at the quality of hardtails there are on the market in the 500-1000 dollar range. Almost all manufacturers will offer a handfull of bikes in that price range. They will typically go from $599, 699, 799, 899, 999, etc. Most of them in that mid to upper price range will share pretty much the same aluminum frame, the difference will be mostly components, and a V pad style brake as compared to a disc brake. Usually the more money, the better the grade of components. You have to decide things like how much and how hard you plan on using your bike. Even the mid price components are very good, if you can afford it, it might not be a bad idea to move up a notch. But unless you are really planning on pounding on it a lot, the best value will be in the 699-899 range. Of course, you can always move up to a full suspension in the $1500+ range, but be careful. All full suspension bikes are not created equal, some are better at other things than others. Recommending one brand over another is tough to do, there are so many good bikes out there. You can get a lot of information online from the various manufactures. Your best bet would be to seek out a reputable bike shop in your area and see what they've got to offer. Establishing a relationship with a good bike shop is going to be valuable down the road, especially if you are going to rely on them to service it for you.