Also GPU miners will make prime coin network more stable and keep it from 51% attack
I think 51% is unlikely, estimates show there are already equivalents of thousands of servers (pretty much all of DigitalOcean amiright?
) mining away. Then add all the miners using desktop CPU at home, laptops even, etc.
I think that not many (if any) persons would have the resources to pull off such an attack unless they devised an extremely efficient GPU miner that was privately released.
Any organizations looking to perform a 51% attack against primecoin would (at this time at least) need to devote huge server infrastructure to to the task. I don't think any college/universities high performance computing clusters would be up to the task (those are sometimes primarily run by 1-2 people and could lead to means of an attack). I don't think any company - even with their own datacenters - would have much interest in a primecoin attack even if they were capable, because those resources are far better used serving clients.
If you want to worry about 51% attack, I think primecoin is relatively safe. On the other hand some (all?) of the SHA-256 altcoins are in a bad place to be right now, eg. terracoin, but that was only possible because of ASIC.