150, yes. I think they're already past their prime. 300? Personally, I think 300 works just fine for archival purposes. It's print resolution, and we're talking about line drawings and simple color screens, not complex photographs. I've done multiple comparisons over the years where I've taken 300dpi scans, upconverted them to 600, and compared them with 600dpi scans of the same page, done at the same time, and found no discernable difference. This was with the intent of restoring those pages to black and white line art. There was no thickening of lines or jagging of edges. Color screens were likewise the same. Fine lines were no clearer/sharper at native 600 than in an upconverted 300.
Perhaps there is some difference on the ultra fine line work of Lou Fine, Reed Crandall, etc., but even then, I've found it hard to spot differences in quality with uncompressed scans. In my opinion, for all intents and purposes, 300dpi is a perfectly acceptable long-term resolution. In the interest of space, the files I upload here have been downconverted to 150 and compressed, but my master files are 300dpi, uncompressed JPGs.
-Eric