RAID stands for Redundant Arrarys of Independent Disks, and it is when you use more than one hard drive together. The two most common RAID setups are RAID 0 and RAID 1.
RAID 0, also known as data striping, makes two hard drives work together as one. This configuration is normally used by gamers and people who want faster loading times or better hard drive benchmarks. Here is a good illustration of RAID 0:
Originally Posted by http://www.pcguide.com/ref/hdd/perf/raid/levels/singleLevel0-c.html
RAID 0 will give you a speed increase, that will be noticeable in Windows loading times and during game loading. But you will not notice any in-game improvements. Also, RAID 0 is not fault tolerant, meaning if one hard drive fails then everything on the other one is useless and all of your information will be lost.
RAID 1, also known as data mirroring, is another form of RAID in which the data is mirrored to each disk. So that you will have an exact copy of everything on each hard drive. RAID 1 is fault tolerant, meaning if one hard drive fails then you still have everything on the other one.
SATA, Serial-ATA, is a new form of hard drive interface which allows the hard drives to run at 150Mb/s and now even 300Mb/s. This makes the hard drives faster, and also you can finally get rid of those ribbon cables! The SATA cables are small little cables that are a lot better looking inside a case.
And the only way a hard drive would limit your overclock would be if you're SATA ports are unlocked, but you have an IDE hard drive (I think?) so that shouldn't be a problem. Also, I'm sure at least the nVidia SATA ports are locked on the Ultra-D.