I would upgrade to an SSD for your OS and keep the Velociraptors for storage.
I've benched the Western Digital Caviar Black vs. the Velociraptors and the MB/s transfers aren't that much better. I've also benched the WD Caviar Black with 4 SSD's and the transfers aren't that much better either.
DDR5? DDR3 RAM was released 6 months ago... Maybe you're referring to GDDR for your GPU...
I have an EVGA GTX 470 (Fermi) and that car just screams, no need for SLI in my system.
Maybe even swap the MB, get SATA 6 GB/s and USB 3.0 - I would definitely do that.
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