I'd recommend downloading the rom manually
booting into recovery.
Make a backup
Wipe cache
Wipe dalvik cache
Install rom from .zip
If it doesn't work, restore your backup and do the same process again but wipe data also.
The moral being the weekly MIUI updates can generally be installed without wiping the data.
I personally don't update MIUI at all, I find a version that works for me and stick with it unless an update comes out that has plenty of new features.
I'm still on the version from about 6 weeks ago. And then even when I update, I won't go to the newest one unless I've seen plenty of reports of it being stable. MIUI has a tendency of having very buggy updates and patches needing to be put out to make it stable.
Check out miui.us for the weekly changelog and bug report. Look in the Droid incredible part of the forum for other users feedback.