I'd be open to upgrade if the cost-benefit ratio is appropriate.
I'm hesitant to offer suggestions for upgrades without knowing what you want to do. Really, it sounds like the MBP is more your cup of tea and you really don't have a need for the MDD.
Just to give you some rough ideas, though:
For about $25, you can get a dual 1.25 GHz module from eBay. Since you have the slower motherboard, you'll have to either...
1) Adjust the bus timing ratio of the module so that it will run at its rated speed on your 133MHz motherboard. This involves relocating some surface mount resistors, but it's not as difficult as it might sound and I can tell you how to build a soldering iron if you don't have one.
2) Modify the motherboard to run at 166MHz. This is done by simply removing one resistor from the motherboard. However, it will require the additional cost of PC 2700 or PC 3200 memory unless you happen to already have this installed.
Inserting a dual 1.25 GHz module without any modifications is also an option, but it will only run at 1 GHz. Of course, fresh thermal compound is also required whenever the heatsink is removed.
A nice video card can also improve performance. Depending on your photo apps of choice, a Core Image video card can make a more dramatic difference than a CPU upgrade. An economical choice is a Radeon 9800 Pro. You can find the PC version on eBay for $20 without much trouble (you'll need to flash with a Mac rom) or one that has already been flashed for $60 to $80. In either case, remove the heatsink and apply fresh thermal compound to the GPU (you have no idea where this card has been).
If you're planning to sell it, the only things of real value are the PSU & ram. The slower motherboard and CPUs don't really sell. Donating it to someone who needs a computer, as you suggested in your original post, would also be very nice. There are plenty of folks out there, many of whom are struggling just to buy food, who would be absolutely thrilled to have it.
