How hot is too hot for my macbook?
90˚c is typical for that system under high load; anything > than 95˚c is when you might want to be concerned. If the CPU gets too hot, or starts to overheat, the system (typically) will clock down the CPU to prevent overheating. The fans shouldn't wear out, so I don't really see any point in trying to replace what's not broken. Intel Mac's run hot, and the fact that the only fan is an output fan doesn't help with heat dissipation. 

Something you could do to help alleviate some of the heat is to download smcFanControl (free) and adjust your fans RPM. Instead of the standard 1800 RPM set it at 3500-3600 RPM, which should help cool down the temp possibly 2-10˚c. You might notice more noise from the higher RPM, although you shouldn't worry about it — MacBooks fans generally max out at around 6,000 RPM, and I've cranked one up to 8,700 RPM before.