Hi, and sorry that I haven't caught up with this thread sooner. In regard to your first question about saline versus silicone, it's actually a bit premature. Silicone implants are extremely natural, but they're only available as pre-sealed bags which have to be inserted into the breast pockets at full size. This means, of course, that silicone implants can't be a whole lot bigger than the size of existing breast pockets (which at the moment you don't have!). There's also an FDA problem about obtaining silicone implants for a first time BA surgery patient. By law, silicone implants are only supposed to be available for a second surgery (ostensibly to correct rippling problems, thin breast skin problems or other conditions which caused a first set of saline implants to turn out less than great looking). A far better approach, as Strider suggested, is to go with a set of adjustable saline implants first to achieve the size you want (or achieve the maximum amount of saline that can be fit in the bag), and then switch to silicone.
You also appear to have a bit of a conflict in regard to the size you want to achieve versus the size of your rib cage and the size of your natural breasts. One unfortunate fact about implants is that implants with smaller cc ratings also have smaller diameters. For a girl with a relatively large rib cage (i.e. 34"-36" bra band), in order to get implants with sufficient diameter to fill your chest and make nice cleavage it may also be necessary to get implants with a higher cc rating than required for the cup size you want. The flip side of course is going with the smaller rating implants and getting the cup size you want, but also winding up with a 2"-3" gap between your breasts. You can try an experiment by cutting out paper plates with 4.5", 5.25", 6" and 7" diameters and holding them in appropriate position in front of a mirror to see how different cc rated implant diameters will look on your rib cage. You can then look at some typical implant dimensions at
http://www.mentorcorp.com/breastsurgery/augmentation/cs_ba_prod_round.htm# and see how your chosen paper circle matches up to different cc rating implant diameters.
Adjustable implants are a bit more forgiving in this aspect since you can go with a higher rated adjustable implant like 800cc to get the necessary diameter, but then simply not fill them up to max once you achieve the cup size you want. However there really isn't much chance of this happening, because every girl that I have ever known who had room in her implants for more saline has ALWAYS filled them up!
Your fears of having limited mobility/limited ability to work out are probably unfounded. I work out 90 minutes with absolutely no problems with my 3800cc and 18 pounds or so (combined) worth of string implants. 500cc/2lbs or 800cc/3lbs or even 1200cc/5lbs implants (every 455cc of saline solution weights one pound) are really not large enough or heavy enough to cause you any problems if you are in reasonably good physical shape (which you certainly appear to be based on your pics). The only thing that will really be different is that implanted breasts will tend to be firmer than natural breasts, and implanted breasts won't tend to flop around wildly the way natural breasts do. Instead, implanted breasts will tend to move en masse, so you'll need a good elastic workout top (I actually wear 2 workout tops when I'm running or doing aerobics) to keep them where they belong.
PS - if you're going to try the baggies full of rice size test, remember to put on a snug elastic workout top over the rice bags so that they will be held close to your chest the way real implants will and give you a more accurate idea of what amount of cc's = what breast size. A very common mistake made with the rice test (or size tests which simply throw an implant into a bra cup at the plastic surgeon's office) is to allow a lot of air space which will not actually be present when the real implants are placed under your skin. As a result lots of girls wind up dissatisfied with their breast size after surgery, which doesn't actually appear anywhere near as large as the test. I can't really tell from your pictures whether the rice bags were compressed a bit or whether they were just "sitting there".