I have seen this question asked again and again by many people, including in many linux forums and frankly, the answer is

yes,  if your server load goes above 1.0, certainly you should think about upgrading your server hardware. Watch out for your server load as sometime occasionally, it could be a sudden spike in which case no upgrade required. For any ideal server, you should always try to keep your server load below 1. Anything in 4.0 or 5.0 range needs serious upgrade.

In your linux command line, type

uptime

(or)

top

to view your server load.

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