What's Quicker

I've got multiple hard drives but trying to work out which to use for my OS. If I used the larger ones I would partition off the space for the OS. These are the hard drives I have (all Hitachi Deskstars)

2x 250GB hard drives with 8MB cache
1x 500GB hard drive with a 16MB cache
1x 750GB hard drive with a 16MB cache
1x 1TB hard drive with a 32MB cache

I was thinking of using the 2x 250GB in RAID0 but unsure how much difference does the cache size actually make?

It's not just about cache size, the 250GB drives being older (I assume) will be slower regardless.

The 750GB and 1TB drives will have much denser data so sequential transfers will be much faster given the same spindle speed.

Best bet is to get a benchmarking too and actually test them.

HDTune is good: http://www.hdtune.com/

I was thinking that but was unsure if the Raid would overcome the age and cache size.

Perhaps, but I personally doubt it.