I have quad G5 connected to Xserve Raid. I am buying a new G5 and want to use the same xserve raid with it. (as you would guess, I am working with uncompressed HD) I know that I would need a Fiber channel switch for that and Xsan software and xserve. But I was thinking if I could just use the the xserve raid over the usuall network? So do you think it is possible? or should I drop the idea?

I have no direct experience with this storage management product, but Fiber Channel should beat Gigabit ethernet throughput by 2 to 8X at least. Fiberchannel is optimized for big streams and is fully bidirectional but Gigabit is optimized for "bursty traffic" under TCP-IP as a normal network port.