Artillery can help, especially with infantry as Pacioli points out. However, as VMXA points out they aren't necessarily, and you shouldn't sweat it. If you play on a pangea, there's no question in my mind... always 4-cavalry armies. They rarely ever lose, and they can even defeat fortified infantry often enough (even on a hill sometimes)... though it's no guarantee as VMXA points out. If you need to ship armies overseas you might want some 3-cavalry armies.
If you attack healthy infantry with an army though, make sure that it has a full 16+ hp or maybe 15. But, if has any real trace of wounds (1 hp doesn't count, 2 hp is worth considering as wounded, 3 hp is *really* worth considering as wounded in this situation), don't attack with a cavalry army against an infantry. Let it heal and attack soon after it heals.
An 8 hp cavalry army can take out a fortified rifle. It's a *tad bit* risky, but as I recall, not too much (my memory might fail me here). A 7 hp army seems risky I don't think I'd attack with it, and don't even bother consider attacking with a 6 hp army, let it heal (under almost all situations, of course). Better to play a little cautious with armies than too boldly. With 9 hp or higher I'd go for one attack, and then make sure it can heal in the same spot covered by something else, or that it can run away to safety.
For cavalry that aren't in armies, you want to use Pacioli's idea (o. k., it's not really his per se, but it is in this context). Redline units with artillery, then use those veteran or elite cavalry to attack. Artillery helping out armies only really comes as near-ideal if you've used all other cavalry doing something else.