well luca most of the products bought now a days have atleast a one year manufacturer warranty, that of what most retail pc's have. Some products even go up to 3 years or even lifetime warranty. You can extend the warranty on retail pc's i believe, not sure if you can on all computer boughten parts though.
You just gotta know whats wrong with your computer if you have a problem and if you still have a warranty on the product that gave out, call their support and wallah.
as far as the ram stuff goes oriion, i would have to disagree. You do not need a whole lot of ram to run now a days games, heck 2 gigs will do fine depending on your operating system, although 4 gigs will give you better overall performance. 4 gigs is more than enough and can be bought for $50 for most of the mobos out now a days. If you want to go about $20 more you can by memory with better timing. Going beyound 4 is not needed at this moment in time, in 5 years, most likley but not now.
Graphics cards is most likley going to be the most expensive thing that you buy when building a pc unless you dont want to go extremely high end of course.
Do not focus on only two of those things when building a computer though, just like setishock means. Every part has to work together, from the PSU, to your CPU, to your graphics card, to basically everything. Focus on everything when building your pc, not just on what you think is better to focus on, all the little things matter to. You cant put a cpu cooler on a cpu without putting some thermal compound on the cpu first, and ofcourse cleaning off the compound you already had on their in the first place, you can but say goodbye to your cpu than.
For a cpu i rather go with intel, especially right now, AMD is in a bit of a downfall at the moment i think they may run out of buisness sooner than people think, they have cut over 3,000 jobs this year alone. You may have more support with intel incase something doesnt work right. AMD is a bit cheaper but i think intel is more reliable, but its all up to you.