How much of a GPU would you need for this to be practical? My PC is 3 years old, and my GPU mining efforts showed approximately 120 days between finding blocks! Even at 7 days per block (I guess some high end consumer GPUs could do that?), how much would need to be spent in a double spend attack to make it worthwhile?

Presumably it wouldn't be worth it for a 10c transaction, and impossible with a 10,000BTC transaction (as the seller would wait for confirmations), so I wonder what is considered a "safe" value below which you might as well take zero confirmations as "good enough" considering the difficulty of launching a Finney attack.

Or would we expect to see botnets finding blocks, meaning the time between launching double spends could be as little as one every 20-30 minutes (approximate time between blocks for something like mining.bitcoin.cz), and that's what the main problem is?