So I'm building myself a gaming PC. The plan is to take 2 identical hard drives and configure them in a RAID 0 array. The speed of a RAID 0 array is precious to me, and I'm fine with the unreliability issues of the RAID array because I have a 1TB external, so I'll just set up daily backups.
My question is, which HDD is more suitable for the RAID 0 array. Remember, my primary concern is speed.
RAID0 won't do you much good for blocking I/O in games; only one HDD can be used at a time in many cases; called a queue depth of 1.
For gaming, the only solution that's vastly superior in performance, is an SSD. You can use RAID0 to combine several SSDs in a larger virtual disk, but if your budget does not allow this, i would focus on separating system + gaming + data disks instead.
For gaming, using a Velociraptor may be faster than two 7200rpm disks in RAID0. But if you have the choice between these two, i'd opt for the 500GB platter one. Cache size is not that much important.
If you go SSD, take a look at Intel X25-M G2, its just a little more expensive than other SSDs but superior in some fields and unsurpassed at the moment. With two of them in RAID0, you get 2x80GB or 160GB, which could be enough to host the games/apps you need the most.
Another HDD could then store the stuff you don't access that much, so you're not limited in capacity. So a setup could be:
System disk: 2x Intel X25-M 80GB G2 SSD in RAID0 creating one virtual 160GB disk
Storage disk: 1x 1TB 5400/7200rpm HDD for additional storage
Or an additional Velociraptor if you want the additional storage to be reasonably fast for applications/games as well. But of course all this depends on your budget.