overclocking my GPU

First, my system:
q6600 @ 2.4GHz
4GB DDR2-ram @667MHz
ASUS EAH3870 @GPU:777MHz, DDR4-RAM:2252MHz on a PCI-E1.0-slot with custom cooling already installed (I'll take a picture later)
A very bad Acer-mobo, probably custom, 'cause I can't find its specifications anywhere

Actually, I only want to overclock my GPU and I want it to have quite a long lifespan (at least four to five years).

1. Should I oveclock?
2. I read that it could damage your GPU and shorten its lifespan. What causes that? The extra stress on the card or the higher temps?
3. How high should the temp go on my GPU? Its peek is currently 68°C (~154°F). I tried to overclock it a bit and the temp stayed the same, but my fan speed went up automatically.
4. How much should I overclock my GPU? When I run the Catalyst Auto-Tune, it went up to 900 and rising when I manually stopped it (it would've gone to 100% fan speed and then it would probably begin smoking, but I don't like that)

I guess its fine since ATI gpus can withstand hotter temperatures the nvidia gpus.nVidiaGaMer

Well.. both ATi and nvidia could handle 68c.

I overclocked it to 801 and 2998 (the slider wouldn't go on the numbers I wanted). Is that too much? Because I saw on the internet that most 3870's are at 800 and this one even has custom cooling.liszt_rhapsody

No, it´s perfectly fine, if you don´t get artifacts when your in-game or unrenderd objects your fine.

isn´t 2998mhz on mem VERY high?, if your´s can go that high you have a O/C winner!

i´d let the mem stay below 2600, most clockers cant reach higher on a HD3870. (1300mhz dual-data-rate)

Rivatuer or ATi tool let you go higher than CCC.