This question can really not be answered. A lot depends on what you do with your data, and what your hardware looks like. For example, you say:
As tables get larger, functionality
like "Table and Index Partitioning"
would be vital to guarantee
This is not true. This function is necessary to allow distribution of data over more IO sets than before. In order to separate IO laod between different disc sets. But when you need this really depends on your hardware and load. For example, with high speed RAID 10's, and distributing a filegroup over let's say 9 groups of those, with8 discs each, you could have 9 data files that run separate IO... in a 4 unit rack case. This is a LOT of io work to start with. I dont think you have an idea about the dimensions Enterprise targets. I know a 190 disc database that does NOT use those features because they are not yet needed there.
More important things are for you:
* The ability to reindex in the background, as reindexing can take hours.
* Some backup / mirroring ability.
For the rest:
What strategies should I use to
determine if the size and usage of my
database requires the Enterprise
Edition Scalability and Performance
You want to play with the big toys, behave like one. Basically start buying a test environment to make real data tests and find out how the situation is in your concrete scenario. Then buy another 2 systems for production,standby and use the original one for development.