I have had bettas for a while, Got my first one when I lived away from home for a year after finished school. Had him in a little tank, and he seemed OK, I was actually surprised he survived, He was in the little tank when I got him and on the way home about 20km, had a sheep run out in front of me and the tank fell over, he was on the rubber floor mat in a puddle of water for about 15km! Grabbed the tank as soon as I got home and filled it with water and he just started swimming like nothing was wrong! Amazing little guy! A friend fell in love with him where I lived so she kept him.
After that I got a 10G tank and had another one in that, just with a few tetras and a catfish. He was fine, now he's in my 55G comminuty tank with black widow tetras, rummynose tetras, serpae tetras, neons, a silver shark, some corys, a couple of silver dollars, a couple of clown loaches a brislenose cat, a gibby and a tandanus catfish. He loved having the extra room, the black widow and serpae give him a hard time sometimes, but he keeps his own and gives as good as he gets. He still has all his tail, so he's doing fine, and they are fun, he does have a great little personality.
Anyway Oscarfishlover, you can keep them in a little tank, but as you said, filtration is best, I used to do a 75% water change every day when I had him in the little tank, didn't think it was fair otherwise, or he's swimming in his waste!
Sorry for the ongwinded reply!
Cheers!

I had four tetras for a while in very small tanks the pet shop tols me to have. They didn't last long! They got fin rot and fungal infections.
A keeper told me to keep them in bigger tanks, with or without tank mates, heated and properly filtered!
Stupid people that don't know better selling pets that won't survive!!!

Bettas can be kept in smaller tanks and even bottles. As someone who breeds bettas i keep mine in 3 litre coke bottles and change the water several times a week. They can be kept in all sorts of things from vases to plastic washing up bowls so long as the water quality doesn't suffer of the smaller amounts of water. Ideally the perfect tank for a male Betta would be 12x12x12 if any body is interested i have made a dvd all about breeding bettas which has some stunning footage of them breeding and rearing the fry check out my Betta site at www.bettasplendens.co.uk