Heighten usually doesn’t do much except increase the DC of a spell. That said, if you want to increase the DC of a spell it’s a great option. It also let’s you spam a spell at different levels if you really like one spell.
That said, I’m assuming you’re playing a wizard (bonus metamagic at 5th). Heighten isn’t quite as good for wizards unless you regularly leave unprepared spell slots.
I’m unaware if fireball’s range is increased, but from what I’ve understood, the effect has to be both variable and numeric, so if range is increased, it would be 400 + 60/level (because the 400 is not a variable). But I’m mostly guessing and would love someone correcting me. Otherwise, if you rolled 1d6 + 5 (caster level) if would all increase because the 5 is a variable based on caster level, and the 1d6 is a variable based on the value on the dice. As to whether you roll a 1d6 and a 1d3 or a 1d6*1.5, that’s up to player and GM discretion. I’d recommend the latter if only for the fewer dice to count.
If you’re 5th level, you don’t have access to 5th level spells. While you do get bonus spells per level, you don’t get access to spell levels to which you wouldn’t otherwise have access. Thus, a 5th level wizard with a 20 int gets +2 1st level spells per day, +1 second and third level spells per day, but because the 5th level wizard doesn’t have access to 4th or 5th level spells, they don’t get +1 4th or 5th level spells per day until 7th and 9th levels respectively. So no metamagic on a fireball at 5th level.
That said, if you’re going for damage, the best options are Intensify, Empower, Maximize, and Quicken(the last two are far away, maybe around 10th-11th at the earliest). Heighten’s good, but you’re probably better off increasing damage. Besides, Spell Focus can usually do Heightens job better (if more specifically)