Once its partitioned, I don't think you will have access to the sd-ext partition from a disk drive stand point. When you plug in your phone and select disk drive, it will register as a flash drive on your pc so you can just copy over the files you want.
By partitioning, you take the limited 512mb of memory the Evo has and basically limit it by the size of the partition. Apps will be stored on the sd-ext partition and the phone will access them through symlinks stored in the phones physical memory just like the app was stored in the phones memory. As an example, my card has a 2gb sd-ext partition on it so my phone now has 2.5gb of memory for apps and data to be stored.