There's no exact data transfer speed that you can expect, it varies from case to case and depends on a number of factors. I assume you could calculate some sort of average, but I don't know.
Theoritically you should be able to obtain twice the access speed and data transfer, although this always isn't the case. I will however say that your overall performance will increase with a RAID 0, but it's impossible to say how much.
I would also assume that yes, you have to use two drives that are exactly the same, as they basically have to mirror one another.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w