f/2.8 ... when lights are low and speed's the need
What you are asking for usually exceeds the normal budget.
Two telephoto lenses come to mind:
On the low end, the SIGMA 70-200mm APO 70-200mm F2.8 EX DG MACRO HSM.
On the high end: the SONY 70-200mm f/2.8 G-Series Telephoto Zoom Lens SAL-70200G
Either of these two will provide for low light like no other lens. Low-light normally dictates a higher cost, so be prepared with the 'plastic'. The best part, you will get more and better shots, like you expect to. Accurate focusing is also improved with the larger glass, but at f/2.8 your depth of field will be really tight. You will need to be "spot on".
Another avenue is to go with a 75-300mm f/4-5.6 lens, but get a much better flash with range to make it work properly. Obviously, there will be times where a flash is either not practical or simply ... "forbidden" ... and that's when these two f/2.8 suggestions come into play. As a class of lens, they can use ambient light far better than an f/4-5.6 lens can. Plus the fact that you certainly would not want to startle your subject with a f/16 flash in her face, during a dismount or some such manuever.
Questions for deeper understanding ...
That is a fair enough question and let me preface it by saying:
The maximum aperture of a lens (in this case, f/2.8) is what will determine if you can use the lens effectively indoors, without the need for additional lighting or flash. Usually, if the lens is f/4 or "slower" (ie, f/4.5, f/5.6, f/6.3, ...), the indoor rule is: Use a flash!
All lenses are designed to basically be used outdoors. The sun is brilliant source of light and can make darn near anything look good, even through mud.
An f/2.8 lens can usually be stopped-down in increments to a minimum aperture of f/22 or even f/32, to be able to get a great "depth of field" or to control the shutter speed you need to get a good shot. You cannot get a wider aperture than the lens can provide.
Stopping-down a lens restricts the light going into it. Normally, your camera's automatic settings will usually accomplish this for you. It will adjust the ISO, the shutter speed, white-balance and the aperture all with a program running inside the camera. Through the settings of each of these, the computer in the camera manipulates them and composes what it believes is the best-looking shot it can make, based on the information it receives from its metering system.
Using f/2.8 is a little trickier, sometimes. The camera's automatic program (or algorithm) really does not like to use the f/2.8 aperture setting as a rule. It usually selects f/4 and counters this setting with other modifications to shutter speed and ISO. The SONY A100's "Super SteadyShot" can certainly help out here, too, with this slower aperture selection.
Normally, f/2.8 requires your focus be "spot-on", because the "depth of field" is so shallow. If you do not fully understand depth-of-field, let me know and I will try to explain it. It is far more important to understand it than when using f/2.8 (or wider, if available by the lens) as an aperture than other narrower aperture settings (f/4, f/5.6, f/8 ...).
Your old lens didn't have a good enough maximum aperture to be used indoors. The new 70-200mm f/2.8 has every capability your old lens had ... plus the ability to take it where the sun don't shine. :rolleyes:
So, you can probably sell your older lens, when you buy the 70-200mm f/2.8, unless you feel the new one (48 oz.) may be too heavy for your outdoor use. It is a lot of weight on your neck. Your old lens probably weighs less than half that.
There's a lot of ground to cover with this decision. You would have the luxury of switching back and forth ... so I'd keep the old one, if your images are turning out just as nice. Shoot some side-by-sides and compare the results. You will see the differences, if they are obvious.
In "Depth-of-Field" discussion
One of the problems or considerations encountered with using f/2.8 or wider aperture settings is focal Depth-of-Field.
Let's assume your subject is a person standing about eight-feet away from your camera.
Simply put, when you set you aperture to f/22 or even f/16 ... almost everything "in frame" is in focus: Close items, distant items and, of course, your subject. It is, in effect, your typical snapshot type of image. They tend to look pretty flat.
When you begin to "open" the aperture up to settings such as f/6.3 or f/5.6, close up and distant items tend to be out of focus (OOF), while your subject is still in focus. You begin to get that "depth" feeling to the image.
When you get to f/4 ... typically anything 1 foot away from (either in front or behind) your subject is pretty much OOF. The image definitely looks well focused, at this point, with a tight depth-of-field.
At an aperture of f/2.8 ... only the subject is in focus and everything in front or in back of them is distinctly OOF. If you misfocus on your subject, you can probably just imagine there is little room for slop at this setting. Accurate focus is crucial.
PRIME lenses with maximum apertures of f/1.8, f/1.4, f/1.2 are great light gatherers, but tend to make your image look "soft" at these settings, as the depth-of-field is so shallow, that only parts of the subject are in focus. In fact, your depth-of-field could be as little as one-inch either side of the focal point (the point of focus on the subject) of the lens. So, you may have just their face, but their ears and the back of their head is OOF. It can make for some very creative photography.
So, hopefully you 'get the picture' of how this may and will come into play, when using the wider settings of aperture. I certainly do hope I explained clearly enough for everyone. Of course, seeing is believing. ;)