Seconding Laki! An adjustable 25W should work fine for you, as would a 10W preset. :) Test it out/watch the temperature for a day before you put a betta in there just to make sure it's holding where it should be. It's really nice to be able to adjust the temperature, so I'd go with an adjustable rather than preset if it was me.
This is my adjustable 25W and it's amazing. :) If you ever upgrade, it'll work in larger tanks, too! http://www.amazon.com/Hydor-THEO-Sub...hydor+theo+25w
Addendum: As far as water changes, every bowl/tank is different, but you might have to do more than 2 50% a week. The only way to really know is to test the water for ammonia and base your schedule around keeping it down. With no live plants, I think you will have to do 1 50% and 1 100% per week.
If you have space enough for something larger than 2 gallons, this is a great tank and can be cycled. It's 5 gallons and comes with light and filter for $25. I have one. :) http://www.walmart.com/catalog/product.do?product_id=14660258&adid=bzv_fb_revshr_ 001