in case of non-reference GPU's you need at least three high-performance[in CFM's] fans in both front, rear and side covers of you PC cases.
usually 120/140 fans with CFM above 45[really measured/benched, not just "printed on the box"] is more or less enough for one card.
and no, 85'C for long-tern usage, lead to not-so-fast, but unavoidable GPU degradation.
but thats just personal trade-off between noise/comfort and hardware TTL/reliability. if you value comfort more, fine. if not, try keep temps under 75'C max.