http://en.wikipedia.org/wiki/Standard_RAID_levels
There can be performance increases with some kinds of RAID because multiple copies of the data is stored across multiple drives. This means that the computer can retrieve data from the drive whose read-head is closest to it.
It would have a significant impact on your system, specifically you'd have to reinstall your software to the new RAID configuration.
Using RAID1 on all three drives would reduce your data cap to 320gig, and triples the data-write time, even though you could better than double your read times you would not want to do that - probably you'd RAID the two big ones and keep the 320gig seperate. This provides a total capacity of 820gig (500gig in raid1). Read performance on the big drives, though, can be as high as 1.5 to 2x
However, if you have error checking or similar features enabled for the drives, performance could actually get worse.
With RAID0 you can join all the drives together to make 960gig, (again, RAID the first two for a 1TB drive) but if you deal with a lot of larger files, like media, then you won't notice any performance increase. You also lose reliability and, in the event either drive fails, you lose all your data on both drives.
The thing to remember about RAID is that it is not intended as a performance enhancer - that is a side effect. It is intended to help simulate large expensive drives using lots of old cheap ones instead. This does not stop home users joining two expensive drives for storage space or as a kind of "backup" system.
If you go to an all linux system (encouraged - of course) you also get the fun of LVM and linux software RAID to add to the mix.