HP? not sure, it depends what Pokemon you plan on using, an obvious one would be Blissey/Miltank. Try looking on Smogon, they have some brilliant ideas.
And it is a lot - say a Pokemon has 300 Atk at Lv 100 at average. If it has an attack-raising nature, that'll increase to 330 at Lv 100. If it's been fully EV-trained, you've given it a whopping 393 Atk. You raised it by nearly a third of it's attack, just by EV training it and making sure it has an Atk-raising stat.