How many high-end GPUs is realistic for a botnet?

I am not sure, but to work with a really small, but easily available sample, my work computer, which was bought about a year ago and not optimized for having any sort of graphics card, came with a ATI Radeon HD4670, which according this hardware comparison, is within a factor of 20 as powerful at bitcoin mining as the best GPUs on the list. I nonconfidently (I would consider additional data strong evidence) expect a significant proportion of computers in a botnet would contain similar GPUs. It's not clear to me how big the Bitcoin community is in terms of computing power (can this be estimated by current mining difficulty?), or whether a botnet could overpower it, but I wouldn't dismiss the possibility because of GPUs.

The moment the spoofing begins, every honest node is being lied about and knows it. This would make the community aware that half the computing power of the network is being provided by dishonest nodes controlled by some particular party. This in turn would create incentive for honest bitcoin users to purchase more specialized equipment to compete against them, or for additional botnets to attempt the same thing (which would grow progressively harder for as long as they do not cooperate). In short, it isn't something that could be done subtly.

My initial concern was based on statements on the Bitcoin website about the assumptions required for security. I am not able to find the page where I originally read that, which explained what an attack would look like. I have found this, which mentions the vulnerability in passing, but also mentions another exploit a botnet could more easily take advantage of, by controling the vast majority of the nodes in the network, it can isolate individual honest users and make fake transactions with them.