I want to start mining bitcoins, so I am looking to buy some GPUs.  I want to get 1.5-2 Ghash/s, but I have not decided yet whether I should just get two or three powerful cards like 6970 or maybe I will get a bunch of weaker cards.

If you want to sell your card, post your offer here.

If you have advice for setting the cost efficient rig, than your reply is very welcome too.

There are better things to worry about then mining botnets. Here are the simple reasons:
1. Most computers do not have a decent GPU that is needed for mining, so GPU mining botnet is not viable.
2. CPU mining is about 2 orders of magnitude slower than GPU, so the botnet with thousands of nodes would be less powerfull than most rigs of the casual miners.
3. Infection is easily detectable because of the spinning fans, thus modes would live very long.

The amount of trolls with 199 posts is even more absurd.

As of now, the profit margin of GPU margin is far too high for people too stop mining. So once you've made the initial investment to set up the rig, it would take a very steep drop in price to make people stop mining. So it does not directly affect the price.

On a second thought, may be bitfen would be more correct.
As in 比特分。

Bitmao.
As in 比特毛。

Linux.
Because it is more secure. And security matters a lot when dealing with bitcoins. Better start learning linux now then wait until some your windows machine is compromised and bitcoins are stolen.