The overall architecture looks good. The report server doesn't get hit very hard, so this one doesn't need to be as large as the rest. Just ensure you're using a solid storage technology with lots of cache and lots of spindles and a high-bandwidth connection technology like fibre-channel or scsi rather than nas or iscsi. We prefer to load our DB servers up with as much memory as possible, usually more than the other tiers.
As a reference, for our medium customers, we're utilizing servers with specs like:
2x Intel 5355 Quad-Core Xeons (8 cores total)
For our large customers, we're using servers with specs like:
4x Intel X7350 Quad-Core Xeons (16 cores total)
For even larger customers, we utilize custom configurations that utilize load-balancing, OTM SCA (scalability) and other methods to scale the load out.
Just remember that as your volumes grow that OTM takes a fair amount of tuning. You'll need to tune the DB, Java, WebLogic/OAS, Tomcat and OTM specific threads. Some of the largest performance gains, though, are around the functional config -- so having an experience implementation team will help ensure that you don't have poorly configured queries and Agents, which bottleneck the system.
Unfortunately, it's too much to go into here, but you're on the right track
Good luck!
--Chris