This thread may be of interest to you:
Travel Passes for Visitors. It sounds like you want to buy the Swiss Pass or Swiss Flexipass, which are available in Switzerland as well as from agents in other countries. In contrast, the Swiss Transfer Ticket and Swiss Card are only available from agents outside of Switzerland, they are not available from within the country, but it does not sound like this is what you want.
The prices in Switzerland and from abroad are set to be pretty similar, but if the exchange rate has changed since the prices were set then you may save a small amount of money. Depending on where you enter the country and when, there may or may not be an agent available at the station to sell you the ticket, or there may be a line of people waiting. Therefore, obtaining the ticket before you get here might be easier.
The part of the thread that I linked to above that is important for you is probably this (note that the prices are from 2010, I haven't updated it with 2011 prices yet).
Swiss Pass and
Swiss FlexiPass
Both these tickets allow for a certain number of days of unlimited travel, and also give free entrance to over 400 museums and exhibits, anf 50% discount on many mountain-top cablecars. They are only available to people with a permanent address outside of Switzerland, but in addition to being available from agents outside of the country, they can also be bought within Switzerland at most major train stations. Before the passes can be used, they need to be validated at an SBB ticket counter. If two or more adults will be travelling together, the second and further tickets can be purchased at a 15% discount (an option called the 'Swiss Saver Pass').
The Swiss Pass is valid for 4, 8, 15, 22 days, or 1 month of unlimited travel on
consecutive days. It costs 260, 376, 455, 525, or 578 CHF, respectively. These prices are reduced by 25% for people between 16 and 26 (a deal called the 'Swiss Youth Pass').
The Swiss FlexiPass is valid for 3, 4, 5, or 6 days of unlimited travel within one month,
consecutive or non-consecutive, with all other days between the first and last unlimited-travel days at half fare. It costs 249, 302, 349, or 397, respectively. There is no youth discount.