Changes made to Apple hardware that are not authorized by Apple may void the warranty. Accordingly, you can expect your service provider to charge you for service to that product if it requires service after the modification has been made. Further, if the modification alters a part so as to make it unfit and ineligible for parts exchange with Apple, a new part must be purchased. Note that pricing is typically significantly higher when parts cannot be sent to Apple for exchange.
Adding DRAM, VRAM or other user-installable upgrade or expansion products to an Apple computer is not considered a modification to that Apple product.
For more information, please see the following Knowledge Base articles:
Article 30284: "
Apple One-Year Limited Warranty: US & Canada"
Article 58127: "
iMac: Modifying Logic Board Voids Warranty"
Article 17402: "
Power Macintosh: Oscillator Chip & Warranty Implications"