It also depends on if you use proxies, do you make SEO indexer check if the sites are still alive and if you run other software or not. On that specs you can push it much further than the 100 threads but you will never know how far until you are there. I did push it on a crappy vps to 300 without much problem without checking if the sites were alive or not.
That sounds about right. If you plan to push it higher try with smaller batches of links first so even if it crashes and doesn't save the already processed links you don't have to start all over again. Also you may want to think about not using all the included sites but only a fraction of them. Filter out those that have less chance of helping you and concentrate more on those that more likely to produce results.