I'd say just keep it. There's no point in closing it. If you're not active, it's okay. I'm hardly on, and when I am, I'm hardly on my priest. It has good memories, they won't die entirely by closing it, but they may fade a bit.
I'd also say if you want to close it off, just clean it a bit. Kick inactives and people who would like to be kicked. Maybe when we get that big guild update, try to recruit and get new members. I'd be happy to help find new members, and would be really happy to see the guild active again.
Forums, of course keep them. I'm hardly on in game, I have my PMs off sometimes, and I'm never on MSN anymore, so this is really my only communcation to you guys.