The nut on my Epiphone G400 broke the other day. I saw this as an opportunity to upgrade because I'm getting sick of that thing. My price limit is very low, around the 400-500$ range.
I play in a post rock, depressive rock, black metal style and I've always loved the look of Gibsons.
This is my first choice:

I've heard good things about Jackson high end guitars but I haven't heard if their low end have a similar price to quality ratio, are they any good?

Lastly I've heard that if you can't get a fantastic guitar, you might as well keep the one you have. Do you think I should just repair my Epiphone?

Any suggestions would be most welcome.
dude the nut is like a $10 fix at most, so i would definitely say keep it, and I have had several Jacksons in all price ranges and loved them all.
If you have 500 bucks to spend, you can get a kick ass guitar for that! You can probably get a REALLY kick ass guitar if you buy used, say Craigslist or pawnshop haggling.

That being said, replacing the nut on your guitar would probably not cost anything more than about 30 - 50 bucks, the latter if you paid someone to do it for you.
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Repairing a nut shouldn't cost much at all. Almost nothing in fact. If you're set on a new guitar, get your epi repaired and sell it, that should increase your budget considerably, maybe enough to get a used Gibson if that's what you're after. I've heard nothing bad about the Agiles though
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$400-500 should be more than enough to buy a pretty good guitar - you're way above the cheap entry level crap with that sort of budget.

The only good advice in any thread like this is to go round as many shops as you can, try everything you can afford and buy your favourite.

If it's only the nut that has gone on your Epiphone though, you should get that fixed as well. It shouldn't cost much so you'll still have most of your budget left to get a new guitar, and you'll still have your G400 as a backup - you may even find that after a while it becomes your favourite again and the new one becomes the backup for a while. I own several guitars and my favourite changes all the time!!
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Repair your epiphone, its cheap. But I give a +1 for the Agile 3100, ived played them before and they are great for the price. Do both, get a agile and repair your G400. Then you will have a good guitar and a awesome guitar.
Pretty much what Darkdevil said there.
Although I wouldn't say Agiles are good for their price,
they're just really good guitars. Period.
All the hardware is great,
the only thing I can see that would need an upgrade
are the pups in them, if you don't like the tone they have.

For the SG,
get one of those pre-slutted nuts from Graphtec.
They're easy to install and definately improve your sound.

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