Hi Mendi, The lenses are really the most important part of your camera system. If you already have servicable lenses in those focal length ranges, I would save your money and replace them one at a time with higher quality lenses. Those will be basic kit lenses and will probably be no better than what you already have. $300 for 2 lenses is pretty cheap and you generally get what you pay for. There are some exceptions, for instance under $100 for a Minolta 50mm f1.8 is a fantastic deal; but it is a simple lens, and relatively easy to produce cheaply and still maintain quality. When you start getting into zooms, things quickly get complicated and the quality of materials and engineering gets very important. One lens I was very happy with was the Minolta 28-70mm f2.8, it will be MUCH sharper than that Tamron 28-80, and while you lose 10mm off the tele end, you gain a constant and much larger aperture (I'm not familiar with that particular lens's specs, but I would guess that the Tamron will only go down to f3.5 at an absolute minimum and it may only go as low as f4.5). It's still relatively reasonably priced too, You should be able to find one for well under $500 and possibly even under $400, well worth the cost in my opinion. As a general rule, I would try to stick with genuine Minolta lenses, they will usually be of higher quality than 3rd party lenses, but there are definitely exceptions. I'd just figure out what specific focal length ranges and apertures you require and research the options in that area, you may be able to find some deals.
-Jason
