They could have taken Gibraltar and defended against the Normandy invasion so Germany could have concentrated on Russia as Turkey would not have been much help.
Its no fun fighting a war on your doorstep and the Turks would of had that issue the moment they declared war on the Soviets. If the tide turned against the Turks they would of been too worried about protecting their own territory to truly go on the offensive and be of any great consequence. As for the Spanish taking Gibraltar would given the Axis control over the Western Mediterranean and would of made taking Malta a much more easy affair. Also it would of cut off Britain from an easy route to India and Australia, perhaps even causing a nationalist revolution in India. The Afrika corp would of been a much more effective fighting force also as the supply convoys from southern Italy and Sicily wouldnt be continually harassed by RAF fighters flying out of Malta.
Perhaps Spanish soldiers doing most of the occupation and defence duties in France would of freed up enough German troops to of win the war on the Eastern Front. Although that would of left Spain open to invasion also and the allies may very well of taken advantage of that especially if Gibraltar was up for grabs.