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I don't think the best hosting has to be the most expensive, so going by what you can afford isn't the best guideline. If your site is performing well, you have no security concerns, and the hosting company is responsive to support requests, then I'm not sure why you'd need to look for anything else. If you feel you are having trouble ranking well, attracting visits to the site, or converting those visits to customers/members/subscribers (or whatever conversion is appropriate for your business model) then you might want to consider the benefits of a hosting upgrade. Is there a specific issue you are trying to solve?