Edit: Oh, I see....a 21 LITER xD I understand now, lol.
Yes, that tank would be just fine for a single betta.....much too small for tank mates, but no need o worry; Bettas are solitary fish by nature, he or she will be perfectly happy with just you for company ;)
I had a tank like that that survived for about 5 years....until I dropped it in the tub during a water change, lol. It was a great tank, saw a lot of bettas too.
You'll want to ditch the filter though; the current is much to much and in a tank that size it just takes up space....instead, get a heater; unlike goldfish bettas are tropical fish and need a steady temp of 76-82 degrees F.
Lol, bettas are tiny compaired to goldfish....a standard betta's adult size is around 2.5in I belive, where as goldfish get to 8-12+in depending on the species. So no worries there xD your betta can live in that tank the rest of his life.
Anywho, now that thats out of the way.....welcome to the site and, hopefully soon, wonderful world of betta keeping! Feel free to take a look around, ask questions, and take a look at our stickys in the Betta Care section like this one http://www.bettafish.com/showthread.php?t=20058
they have loads of super helpful information for beginners. ;)