where in the world do you live?
people who have lots of bettas tend to keep them in 1 or 2 gallon containers, but if you're only going to have the one, why not spoil it! 5-10 gallons is great, and at the size you could even have a (very carefully researched) companion for him.
i have 2 bettas, 1 is in a 4 gallon acrylic tank with 25 watt heater and 2 fake plants, he loves it.
my other is in a 7 gallon glass tank with internal filter/heater posh stuff, i think he likes it so far! (he was upgraded from a 2 gallon glass bowl on thursday)
depends on your budget really, the 4 gallon + heater came to about £60 in the end (yeah, i got ripped), the 7 gallon was a gift, but was probably in the region of £90-£150.
i also have a 5 gallon hex, the filter is too strong for a VT though, looking for a plakat for it, that was £50 including shipping off ebay.