May I ask why you have a budget in dollars if you live in the UK? Are you buying off a US shop, or...
EDIT: Get two of these
Computer hardware and software at amazing prices, available online from Scan Computers UK
If your going for a striped RAID array then 10K RPM is sinmply not required.
This drive says Max. 167GB Formatted Capacity per Disk
But it's a 500GB drive???!