yea you can mix any hard drives. i tried mixing my 250gb with an old 120gb and it worked fine except it only used 120gb off of the 250gb hard drive, so i essentially ended up with a 240gb hard drive. its suggested that you use identical drives though.
it actually does help the performance a lot, but also remember that it doesnt only depend on what drives you use, but what RAID card you use as well. most motherboards come with a halfway decent one but there are pci raid cards out there that have their own integrated memory and everything...
two hard drives in RAID 0... depending on the drives and the raid card used can give performance close to that of a single 10krpm hard drive, like the 74 and 150gb raptors... and even outperform them in some benchmarks. its definitely not the same type of performance increase that you'd get with a 10krpm hard drive anyways, but you dont really need that much speed anyways, so RAID 0 is just a cheaper way of doing it.
for instance, a 74gb raptor with a 16mb buffer would be at least $150 and my two 80gb drives each cost $44... so say around $100... so i get 160gb, thats twice the storage