Your calculation is wrong. One 1070 is able to produce about 1.6 txs, so 100 x 1070 are able to produce 160 txs. For a full transaction you need to do two time

Your calculation is wrong. One 1070 is able to produce about 1.6 txs, so 100 x 1070 are able to produce 160 txs. For a full transaction you need to do two time POW (send and receive), so if you use the same cards to receive the tx also you are only able to produce 80 txs. Right now I do not think you would harm the network up to about 10k - 50k txs (a simple laptop is able to monitor about 1k txs). So you may want to target at least 10k txs, therefore you would need more than 10k graphic cards at a cost of about $500 each at least, this sums up to $5,000,000 and as I said it is not clear that this would harm the net in any substantial way.