You computer doesn't "need" a drive of a particular speed, just one of a particular type (IDE vs. SATA) as described by techmo34.
Watch out for hype around hard drives and 6Gbit/sec SATA. If your computer has ANY type of SATA port (even 1.5Gbit/sec or 3Gbit/sec SATA) you can buy and use a 6Gbit/sec SATA drive. But don't expect it to work any faster than the older SATA drives. The SATA connection speed isn't the bottleneck - it's the disk platters themselves - so if you're looking to get a fast drive then review it's specs carefully rather than just going for one with a 6Gbit/sec connection.
Also if your 2nd hard drive is for backup, or less rarely used files, you can look at a slower HD, such as the WD green series (lower rotational speeds, less power usage). If you are putting higher performance items on it, then you would look at the higher performing/more expensive drives (although higher cost doesn't EQUAL higher performance all the time).