I played for a couple days on an hd3000, and with everything turned to low @ 1920x1080 it ran decent. If I got into a group though, it slowed down a lot. Especially, if it was a high AoE damage class, like CM Wiz.
As far as Inferno Difficulty, I don't think it is that much easier. Besides the obvious nerfs like Invulnerable Minions and elite packs of the exploding mobs in Act 3, nothing really stands out. The problem is that as a whole, we completely outgear all content. Remember when you first hit 60 and went into Inferno? You got rocked, hard. Now days, that doesn't happen because you go on the AH and buy some cheap gear and you outgear MP1 instantly.
When I first beat Inferno on my monk, I had like 4k dps 900 AR and like 1500 LoH. It was extremely painful and slow.