You're not doing any parity raid with those arrays. You won't really see much of a difference in performance. Perhaps a little for small/burst transactions as you are increasing your cache (128 -> 512MB) but that's rather small as well and depends on your workload.

As alfaunits & myself have 'bantered' in another thread the 8x lanes is not something you'll actually use with the cards (it's going to have the same max cap speeds when writing out to the drives, only going to the cache would have some benefit of anything over 4x pcie.

In your case I would say increasing the size of your array would give you more bang for the buck than getting the card/changing your MB.