Hardware purchase question

We currently are utilizing postgresql on 2 servers with the following
configuration:

2 - 2.4 Ghz Xeon processors
4GB ram
4 36gb 10000rpm scsi drives configured for raid 10

We started out with one server and as we became IO bound we added the
second. We are currently considering purchasing another identical server
to go along with these. In addition to this we are also considering a scsi
attached storage device in the 10 - 14 drive range configured for raid 10
in place of the onboard 4 drives we currently have. Daily about 30% of
our data gets updated with about 2% new data. Our query load is about 60%
reads and 40% writes currently. My question is what type of performance
gains can I expect on average from swapping from 4 disk raid 10 to 14 disk
raid 10? Could I expect to see 40 - 50% better throughput.

The servers listed above are the dell 2650's which have perc 3
controllers. I have seen on this list where they are know for not
performing well. So any suggestions for an attached scsi device would be
greatly appreciated. Also, any thoughts on fibre channel storage devices?

Thank You,
Bo Stewart