Rooting your phone isn't going to achieve what you're looking to do. The EVO doesn't have 800Mb of memory, no matter what you remove. You're going to install your own apps at some point and not all of them will move to your SD card so they space will still be taken up.
If you're looking at custom ROMs, try
Myn's Warm 2.2. You will need to root first and the Unrevoked option will work.