...initial question - did you happen to install Arcsoft? Like, running the uninstall cab, soft rest, install Arcsoft?
Here is the Arcsoft thread:
http://forum.ppcgeeks.com/showthread.php?t=61802 or you can check out the Guide in my signature, that has the specific steps along with links to a couple of different versions of Arcsoft.
You need to run the Arcsoft removal cab, then soft rest the device [this is important], and then install a new version of Arcsoft [links are in the aforementioned threads].
When properly installed, you don't need to go into the registry to adjust the settings, simply:
Go to the Text Tab in TouchFlo.
Press the All Messages Softkey
Press the "Menu" softkey
Scroll down to "MMS Options"
Select the "Servers" tab
Click on "SprintMMSC"
...adjust the send limit from 500K to 1000K or 5000k.
A note, if you go above 500K, you run the risk of some peopel not getting your MMS because they might not be able to accept large files. ...like Verizon users.
Hope that's more informative.