Quite frankly, you sound like you're in over your head just a tad. I'd recommend giving steve @ rack911.com a shout -- he's great with large forum configuration / optimization setups and even handles hardware recommendations.
He's setup a few forums on my behalf.
On the other hand, re: your questions.
In this case due to the high IO I'd highly recommend having SCSI for both systems. While softlayer might seem like a good choice, they're really not. Their infrastructure: great. Their support: piss-poor.
CentOS all the way!

//My two, random cents.