WD's Caviar Blue internal hard drives are the cheapest in its Caviar family, which means you shouldn't automatically expect the best performance compared to its Caviar Black lineup. But they just received a shot of adrenaline in the form of a SATA 3.0 interface upgrade, giving budget buyers access to 6Gbps bandwidth.
SATA 3.0 gives you double the bandwidth of SATA 2.0, while still being compatible with older motherboards that don't support the newer interface. That means you can use the drive now with your current build and then take advantage of the performance boost when you upgrade your system to a SATA 3.0-compatible mobo. (Of course, those with a compatible motherboard can take advantage of the new interface right away.)
According to Fudzilla, 6Gbps shouldn't make a huge difference in the Caviar Blue's performance, but the drives are cheap enough that it might not matter to you. SATA 3.0 support will be available for drives in the following capacities: 250GB, 320GB, 500GB, 750GB, and 1TB. The 1TB WD10EALX drive is $74.99 at Newegg, while the 250GB WD2500AAKX drive is just $44.99.