Symbian OS and S60 aren't as demanding of resources as Window Mobile, so the dual core CPU in the E90 is more than up to the task.
If you want to stick with WinMo, there's the HTC TyTN (still shipping with WM5, I believe). The TyTN II will probably be available from importers within a month for around $800-$900. That's the one codenamed the Kaiser, and it adds a GPS, different slider design and WM6 to the original TyTN design. It's 400MHz, so it will be faster than the MDA (the TyTN is also faster).
-------------------- Lisa Gade Editor in Chief, MobileTechReview