Then you should really read up on raids before you ask these questions about a step by step setup. You will then know how to set them up. There is good documentation here and all over the internet. It will solve all the quesitons you have asked here.
I've never done RAID before. This is my PC specs:
HP DC7900 Core 2 duo 3.0Ghz
4GB 800Mhz DDR2
1x 160GB SATA HDD (HD161HJ) on SATA0 slot
1x 250GB SATA HDD (ST3250823AS) on SATA2 slot
ATI RADEON 6570 2GB DDR3
What I have in mind is a RAID 0 setup. So that I can gain maximum perfomance while playing games like FSX. What are the implications (different sized drives)
The samsung 160GB drive has 3.0Gb/s interface and Seagate 250GB has 1.5Gb/s. Is it the REAL speed of the drive or just the interface format?
Please give me a step by step guide on how to setup RAID.
raid0 won't improve your gaming experience, it might improve load times but that's it.
raid0 doubles you chances of drive failure.
raid levels of all kind work best with identical drives, mismatching drives in raid essentially holds speeds & size to slowest & smallest drive in the array. (by combining a 160g sata2 with a 250g sata1 you'll essentially end up with a raid0 drive that's only as fast as the sata2 is itself and waste 90g from the 250g drive).
raid0 is something techies tinkered with 10 years ago to improve disk performance, not something of much use anymore.