If you have a good RAID card or controller, then try this:

1 ) 300 GB Hd + 200 GB HD stripped in RAID 0 to make a total of a 500 GB RAID 0
2 ) 500 GB HD + 500 RAID 0 mirrored into a RAID 1

You may end with 500 GB only, but the overall reading performance should increase. In most cases this depends on the raid controller because there is lots of raiding in the mix.