On a DSLR like the D50 where you can up the ISO to 400, 800, or even 1600 with minimal noise, gaining one or two F stops (such as 3.4 to 2.8) is not overly critical. More important is getting a lens with low distortion (key) and fast autofocus (if you primarily use AF). The primary advantage to a larger aperture is that you can use faster shutter speeds, allowing for handheld shots in lower light, but the ISO adjustion serves the same purpose.
The 3.4 is going to be at the wide end. A 2.8 compared to a 5.6 means that you can use the same shutter speed in 25% of the light OR shoot at 1/250 when you otherwise could only use 1/60 shutter speed. On top of that your shallow DOF will give your action and wildlife shots that have *POP* in that they object seemingly leaps out of the busy background.
Hands down, there is a complete difference between a Tamron lens and let's say a Nikkor 28-70/2.8 HUGE difference and I'm not talking about just the aperture, I'm talking about quality, clarity, sharpness, speed, bokeh, everything...
I will never go cheap glass again!
I just bought the 28-70/2.8 not even a month ago, and it is just as sharp as my Nikkor primes. I also just purchased the 70-200/2.8.