HQ:
1 Librarian 100
w/ Wings of Sanguinius
Can't go wrong with this choice, in a 500 point list the librarian is a good selection.
TROOPS:
5 Assault Marines 135
w/ Power Fist, Flamer, Meltabombs
This doesn't make sense to me; if you have the fist there are next to no situations when your going to use the meltabombs instead especially in 500 point games; I mean how many monoliths and land raiders are you expecting to see?
I'd very much advise dropping those meltabombs for points to spend elsewhere, maybe in making the flamer a meltagun. Otherwise I would suggest dropping the powerfist down to a power weapon and keeping the meltabombs.
5 Assault Marines 130
w/Power Fist, Flamer
I would actually suggest dropping this squad altogether in place of something else, like a pair of multi-melta attack bikes or a predator or land speeders.
Getting as many bodies in at five hundred points is helpful, but assault marines are short range close combat troops, you really need some other speed and support for them.
5 Assault Marines 135
w/ Power Fist, Meltagun
Wouldn't change the weapons of this squad, though maybe instead of the jump packs for these squads, consider dropping them in favour of rhino's. That way you can take a different power for the librarian, retain your speed, and have some armour protection.
At five hundred points, things are tight and that makes it interesting.