24/7/365 Overclocked CPU - Upgrade or Not?

Hello,

I run a VERY CPU intensive program as a hobby (like folding) which runs 24/7/365.
I got my 2600k to very very good overclocks. (4.9 ghz without increasing the voltages) - only to run into cooling issues. (at 4.3ghz its stable at 75*c (highest core))

My question is: What attributes affect longevity? Temperature? (GET MORE COOLING!) or does just having it overclocked reduce its lifespan.

Bang for Buck, when its all said and done, (keep electricity cost in mind too!) Should i bother upgrading cooling to overclock any further? Will $100 in upgrades net me 10% more CPU?

2600k
Asus P67 Sabertooth Board
ALC Eco modified to push/pull with a (disgust) Silenx 120mm "<74cfm" fan and the stock fan.
Silenx 60mm rigged to the "Assist" fan (the 50mm slot)
Modified case - its essentially not existent (lots of holes) as the CPU cooler is the circulatory system.

16gigs of 7-7-7-20 / 1333 memory (it was cheap)

PS: Currently its in my bedroom, so noise IS an issue. in 4 months ill move and it WONT, so can get screaming 10000 RPM fans (probably change coolers too)