1. Untill somebody tries, there is no way to tell. There have been several drafts of the .n standard, with more to come. It's still a year away with some open questions so I would imagine there will be more drafts before it finalizes. There is a good chance products are compatible, and a good chance they are not (50/50??).
2. Not before people have tried. I have good experiences from Linksys, but they have a history of completely changing the product and only changing the revision of the product (WRT54G for example). I have bad experiences from Belkin so they would not be my first choice (Netgear is not available here in Finland).
3. While the base station on paper supports 500mbit speeds, in reality it will be alot slower. With luck you might get 200mbit (double your current wired speed), but even that would probably degrade when you have several clients on the network (or if the client does not have a perfect signal). Add to this that the latency is higher, and I would probably try to keep the server on a 100mbit cable for robustness (of course you can always try).
4. Apple claims there will be a lot better coverage, but I don't think you should expect miracles. Maby you could expect that the areas where you have medium signal now will get good signal, and the ones with weak will get medium?
5. At least you have the choice to do so once you try. Be sure to let us know, as until now I have not heard of anyone with user experiences. The fact that Apple separately mentions it makes me feel the difference will surely be noticeable.
6. Maby in the next revision... There's some web page that shows the average update frequency for Apple computers, don't remember if it had networking gear too... It's your best bet at least. Generally I would imagine an update every 6 months or so.