Well, you are always "shortening its lifespan" if you use it, at least vs. not using it. But the point is: Will it last "long enough" for you? My guess is probably yes - unless you care about it still in 3-5 years or so from now, when it's hopefully outdated. 70°C chip temperature feels rather low for me, should be fine I guess.

Same thing happens with my card :-)

The aero theme starts flickering with squares etc

Are you using Windows Aero?

If so disable it.. it requires gpu power. When mining it uses all the gpu so Aero can go glitchy, might not be artifacting.

You can try to change your mining program. I have artifacts when i use phoenix miner.or decrease intensity )

Do you do overlocking?

Not shure about 7870, but on my 7850 i getting ~387 kHash/s and i think i can get more (experimenting now). I'm using 2.11.3 cgminer.

UPD. Try to use configs like here (find your GPU in list) https://github.com/litecoin-project/litecoin/wiki/Mining-hardware-comparison . You must get about 400 kHash without any problems.

Mining bitcoin yes this would be correct.  LTC difficulty is much lower and your numbers are definitely off...with that said i do agree it would take a very long time to get your own block.  Pool mining is definitely the way to go.

Basically if you were mining with that card a year ago things would be different.  Today it may take you months or years to get a block. 
Look at it this way - there is a pool which totals 1 TeraHash/second.  It takes that pool 12 hours to get a block and get that 25BTC reward.  Your card pulls 300MegaHashes/second and is basically 3000 times slower than everyone combined in that pool.  So in the long run you will need 36,000 hours to find a solution to a block which is 4 years. 

My numbers are all off by a bit but its the magnitude that matters - you will have to wait years without joining a pool. 

I am also running a 7870 and am getting about 295 kH/s...i cant get it any better than that.
gpu-threads 2
thread concurrency 5120
intensity 12
worksize 256

Good Luck please let me know if you find other settings you are more successful with!!

You have to find a block, so when you are mining yourself, you have to wait till it finds a whole block, you don't get anything till you find the block.

http://www.litecoinpool.org/calc?hashrate=425&difficulty=205.77652398

This is an estimate of what you could be getting in a pool vs. expected probabilities of finding a block. I found the hashrate of 425 over here: https://github.com/litecoin-project/litecoin/wiki/Mining-hardware-comparison

I am afraid your 280 is slower than it should be.

Looking at the numbers, it will take you almost a month to get the reward of 25 coins, $4 and change each, versus $260 a month in a pool.