Opening the case doesn't void the warranty (if it did Apple should have welded all MacPros shut before shipping them LOL). Replacing the hard drives won't void the warranty, replacing/adding RAM won't, upgrading the video card won't. If you damage something while doing an upgrade (hit the motherboard with a screwdriver, zap a component because you didn't ground yourself, damage the PCI Express slot while putting a video card in for example), those are your responsibility to pay for.