I think you will find that you would do better to put money into display card rather than a hard disk. A fast disk will ONLY improve your game load times your FPS will not change.
When it comes to gaming, the bottleneck will never be the hard drive. As long as you have 32MB of cache, anything goes. BTW, 10k and 15k drives have the distinct disadvantage of running slightly warmer than a 7200. And unless you're running a server, you probably don't need them.
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