Warranty Issues
I always recommend that in order to save both time and money you check out the AppleCare Knowledge Base where you might just find a simple answer to your service issue and that you also invest in AppleCare just in case you should you experience a serious hardware failure.
In my experience the operating system is the most likely culprit when and if a unit fails and this is usually the result of lack of maintenance or operator error. If a unit fails for this reason it is almost always a simple repair and often possible to recover data. Operating system and software issues such as these are generally not covered in the warranty but are the most common issues that you can expect to face.
Another common issue almost always not covered in the warranty is hardware damage. An example could be a broken hinge on a Titanium PowerBook. Hinges are not available separately for the Titanium PowerBook, the only option being a complete new screen which includes hinges and can run around $1800.00 without the labor. We can often effect this repair for as little as $290.00 including labor!
Another issue not usually covered in the warranty is damaging the motherboard by upgrading the operating system without first updating the firmware. I have seen this damage happen often as most people are simply not aware that they might need to update the firmware before an operating system upgrade.
If you have any of the above issues and you feel that they might be covered by the warranty, speak to me first as I am experienced in dealing with many of the computer warranty companies including Apple Computer and can usually recommend to you the best approach to help ensure that your expensive computer is covered.
If all else fails I can repair, often cost effectively, any fairly recent Mac computer. This should include most with the G3 processor and almost all with the G4 processor.
Hardware Upgrades
I can recommend and carry out hardware upgrades such as: