Well that depends on several factors as far as how much lift & duration you want/need.
Like Lex said it really depends on your combo? Is drivability, emissions an issue? Do you use a factory based efi setup?
Anywhosit more lift with a more aggressive rate of lift (rather than increased duration I beleive this is termed area under the curve) will probably give a more satisfying increase in power. As this would diminish the trade off in torque for horsepower that typically acompanies installing a bigger cam.
Now wanna see somthing funky! check out valve curtain area? If its a given that the three valves below are saddled with ports that flow exactly the same at the specified lift despite varying sizes then they all have the same valve curtain and effectively flow the same amount of air despite having varing amounts of lift and valve size.
1.84 x .500 = .92
1.94 x .474 = .92
2.02 x .455 = .92
Okay where does this leave us? Well say for instance your heads have 1.84 valves that reach peak flow in the .500 region? That means you are going to have to run a cam with .500 lift to equal the flow that a 2.02 valve would get with a .455 lift.
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