well, you can on a G5 using SATA...I did it no problem straight out of the box....well, it took me a while to figure out how to Disc Utility to let me create the array, but my RAID 0 array has been perfect since day 1.
320GB of 100MB/Second joy.
I'm not sure if you can boot to PATA RAID arrays or not...not even sure it will let you make one...but you're talking about 30 and 40 Gb drives...which says to me that you aren't talking about SATA.
as to the size of the discs and what you get for striping, no, they don't have to be the same size, but it will use the least common denominator. Example:
30gb drive and 200 gb drive will create a RAID 0 array of ~60gb, or a RAID 1 array of ~30gb.
The rest of the space will be wasted. It's possible, I think, to create a partition on the larger drive to offset this wasted space, but I'm not sure that it's a good idea, because at that point, you're taking a virtual drive spread over 2 physical drives and adding another virtual drive's worth of complication into the mix. Talk about asking for catastrophic data loss!
I'm not sure you can do RAID 0 without a physical RAID controller card on standard PATA drives, anyway.