One inch of fish per one gallon of water is a good rule of thumb to follow. Also keep in mind that *most* fish like to be kept in odd numbered schools. Another thing to remember is heat - tetras and other tropical fish need heaters and most small tanks do not come stocked with them. IMO, a single betta should live healthy and happily in that tank you posted (although, like katy said, bigger is always better).

Have you by chance looked at Wal Mart for fish tanks? That's where I bought my 10 gallon tank and I got a pretty good deal on it

http://www.liveaquaria.com this is very informative website. It lists many types of fish, their temperaments, what PH their water should be at, what temperature their water should be, etc. You can also buy fish there, but buying fish online just seems a little freaky to me!