My website used to get around 4K unique views a day and never had any problem for 4 months. It took my host to suspend my account in 4 months. Mainly it was due to Database load in other words CPU. I suggest you use W3 Total cache because it has DB cache option and set the DB cache to a greater time. It will help you reduce DB load, also check if you have unnecessary queries like too many related posts, too many sidebar widgets and items in them, too many tags in sidebar etc. In short, reduce the number of queries. There are many plugins out there to count how many queries a page produces. I believe 50 queries of a wordpress page is a safe number. Best of luck 