i am fitting a new hard drive in an existing server next week and am going to expand the raid array.
We are running ESX 3.5 on Linux and was wondering if there was anything else i need to do in ESX itself to make it aware of the new space available
Your options are to create a new datastore and keep it separate from the existing datastore, or to use an extent to merge the two datastores together as one. You'll see a lot on these forums about the dangers of extents but I'm not all that afraid of them. if you're looking to keep one single datastore then an extent is the way to go. Otherwise create a second datastore and balance out your virtual machines across both.
Check out the following thread for some instructions how to create an extent to expand your VMFS partition.