Here's a little discussion that may be of some help:http://www.computerforum.com/151083-just-got-my-cm-v8-hsf-thermal-paste-question.html
I'm using Chill Factor II TIM on my CPU, it seems to be 3-5C colder than the CM V8 Thermal Compound. But that can just be the method in which you apply the thermal paste.
All in all, the V8 is a pretty decent cooler, as well as the thermal paste, so I wouldn't worry if you whacked some on your GPU.
As for any form of upgrade, I would not personally suggest it. Nor would I suggest worrying about 70C, that is not hot. Hot is 85+, that's when you know what you are doing is fully stressing the card to the point where it could be unstable (9/10 times is still stable).
In my past experience with computers, I spent £800 on an AMD gaming build with a GTX 470, and £1500 on an INTEL gaming build with a GTX 580, the difference is unnoticeable in some games, I should have waited. But the want was too much, and I gave in.
I had a £40 NZXT Gamma, and upgraded to a £140 Silverstone Raven RV01B-W, and the temperatures increased.
I've wasted so much money it is unreal, at the end of the day if you want something, it's always better to wait, cause something better will surely come along. That HD 5770 is good for another year or two. Then it may start to struggle a lot.
But by that time, as I said, the new cards will be coming, 28nm! Can't wait to see some benchies.
Hope this helps, and if you're thinking about upgrading any part of your build, don't, you'll find it will be a big money sink, in a years time, you will thank me, and you'll have bought a whole new rig, with all the bells and whistles. If you upgrade now, you're stuck using the same components, as I know for a fact spending more money on something makes you more attached, and more likely to go even more insane, and buy another 4gb RAM, an SSD, a new graphics card, processor and motherboard (it definitely gets expensive)! A prime example (World of Warcraft) lol.
So if you're worried about the best bang for buck, stay away from upgrading little and often. And bear with what you have for now.
So yeah... I'm waffling.