As fas as the connectors in the front, they are not proprietary and we just needed a longer USB Cable - I picked up a 24 inch one that solved that problem. (was about 3 inches short with the stock one)
I hope this thread is not too old...
I'm also replacing my Dell 435T's motherboard, but with a Gigabyte GA-X58A-UD3R
You mentioned that the front panel connector was not proprietary (or did I misread that?), but the 435T connector is different from what the Gigabyte expects. Do you know what the pinout for the 435T's front panel connector on the motherboard? I guess, if necessary, I'll have to trace out the connections myself and (possibly) rewire the connector so it matches with the Gigabyte's pinout. :-(