No Such Thing as a Free Lunch
Running RAID 0 will not cause any reduction in speed of your Hard Drives. But I can not see that running 6*74GB Velociraptors will be worth the CBA of using 1*300GB Velociraptor.
A marginal increase in average access times of 3-5% might be seen and your O/S might load quicker but the rest of the PC hardware is then going to let you down. If you had a Mini Computer board with Multiple Data and Address Busses working Asynchronously/Full Duplex you would indeed see a greater improvement in access and load times. But we are stuck with the Dinosaur of Common Bus Architecture that is the PC!
Yes RAID 0 is very good with Very Large Files but is poor for many Small Files, not to mention setting your Stripe Size to fine tune the system.
If you wanted RAID 0 I would go with the 2*150GB velociraptor option under the rule of KISS.