The 5870 is still a surprisingly strong contender for gaming performance. You would be well served by trying to extend the life, either via factory replacement (you have to trust them to get it right), or by doing it yourself (you have to trust yourself to get it right... but you're right there if you need to blame someone
On the other hand, 28nm parts are 'around the corner'. Which 28nm parts is still up in the air... could be low end, and low/mid range, could be mobile parts. Nobody's talking.
Have you tried cleaning the card?
Here's a relevant question. How comfortable are you with assembling your own systems? If you can do that, you can replace a gpu cooler.