I don't know, but I will say that the server was running pretty smoothly at night (after 8:00 pm EST).
The problem with upgrading the server is probably that its a recurring cost, and every sale of merchandise is only a one-time source of income. He doesn't have any constant revenue streams from the site that I can see: No ads, no subscriptions, etc. If I had to bet, I'd say he's probably at the limit of what his income from OOTS can afford already. There are just orders of magnitude more people who want to read the comic than will ever buy a book.