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That being out of the way, you do not want a "cheap" set of pistons.
They are one of the engine components you do not want to scrimp on.
Get a good set of forged pistons that will hold up to any future modifications you might make.
There are no cheap changes to get the most out of your engine. browse the sticky thread "In the Beginning" to get an idea of where to start.
The best mod you can make that will result in the most HP is an HCI or Top End kit.
Trick Flow makes one that advertises 360 HP by installing their kit.