Skill > gear any day for getting good shots. with that being said, i tend to use an old Canon FD 300mm f/4L that has been adapted to k mount as my primary lens for soccer. If you got the budget, check out the Sigma, FA* and DA* equivalents, as you get autofocus with those lenses as well. As much as i'd love to have any of those for myself, i just don't have the funds
On my second body, usually my roommate's k-x, i have an M135mm f/3.5 for when action gets close. this setup i use is fairly affordable, but requires a bit more skill as it requires that you know how to use fully manual lenses.
as for the tamron 70-200, i havent had a chance yet to shoot with that lens. i have, however, shot with the sigma hsmii and found it quite the competent lens for sports. With that being said, i'd still only use the 70-200 as a backup for the 300 when shooting soccer. For basketball, you can get quite a bit closer than soccer, so you can most likely make do with the 70-200.