4 Cavair Blacks in a RAID

You'll get high transfer rates and more concurrent I/Os per second, but the access times aren't really any faster than for a individual drive.

In general: it'll help, but don't expect it to rival an SSD.

You will notice that your load times will decrease.
Access times may become longer due to system overhead of having to read from 4 drives.
Personally, I ran RAID0 for years and years, but not any more. Drives have gotten fast enough (though still the slowest part in your system) that I don't believe RAID is that big of a benefit anymore. You can just drop in a ton of cheap memory, and your programs, games, etc don't have to hit the hard drive all the time like they did a few years ago when we had to do with a lot less memory.

It is a lot of storage for the money though, and a LOT of stuff to lose if you break the array. It's your call.

how low for a 750mb caviar black? I never knew that they made 750mb models lol