Having owned and managed a couple of large forums on varying interests I feel your pain. We ended up going for a 'members area' where if you paid £x a mount a year in subscription it gave you a seperate area with certain perks.
Subscriptions are easy to setup in the software with a paypal option (we offered cheque payments too but this became a pain).
Adding the new area costs nothing and it makes the donations/payers feel like they have got something for their money ....
just an idea
