I mainly go to bars with a good and/or huge beer selection.
It's not cheap, but it's good
A place with nice food and a good beer selction that is the closest to Metaltown is "Ölrepubliken".
http://www.olrepubliken.se/
There are 3 bars in the "Bishops Arms" chain of pubs in the city,
the closest one to the festival being "Plaza / Västra Hamngatan"
http://www.bishopsarms.com/Goteborg_Plaza_Presentation_DXNI-744_.aspx
the more central located at Avenyn "Park"
http://www.bishopsarms.com/Goteborg_Park_Presentation_DXNI-701_.aspx
and one slightly more "off" from the centre, but located more around the bar streets "Järntorget"
http://www.bishopsarms.com/Goteborg_Jarntorget_Presentation_DXNI-50689_.aspx
The food on these 3 are similar, the beer selection varies a bit. Avenyn has some cask/real ale if lucky, Järntorget focus more on stout/porter for their guest taps. The BA pubs tend to be rather pricey, but with high quality.
One bar that is awesome for a nice session with great beers is "3 Små Rum", not far from Avenyn. The food is more to the Persian style, the taps focus mainly on Swedish micro. Huge bottled beer selection.
http://www.3smarum.se/
One of the best value food selections and the most educated/knowledgable beer staff in town you'll find at "The Rover" on Andra Långgatan, not far from Järntorget. Also a big whisky selection.
http://www.therover.se/
One of the cheapest places for good ber in town is "Haket", right next to an awesome Japanese restaurant (Yamato) with the same owners.
http://www.haketpub.se/
If looking more for "metal places" I'd say that "Rockbaren" would be the best one. Centrally located, food is nice as far as I know, the toilets are finally renovated and they do have at least one good beer.
http://www.rockbaren.com/
The club "Sticky Fingers" are doing afterparties for both Friday and Saturday after the festival.
http://www.stickyfingers.nu/
Let me know if you need any more info, I guess all the websites listed are in Swedish.
You know where you'll be staying hotel/hostel wise as well?