Emos, i completely understand your upgrade situation. i dont know if i specifically mentioned the model Dell i used to have, but it was a Dimension XPS R350 (same as yours, only w/ a 350 mHz CPU ass oppsed to your 400 mHz). so when i upgraded the CPU, i also had to get a new mobo. and Poof is on the right track about the power connectors. i dont know about the main mobo power connector, but i do know that the front panel connectors in the front of the case (the ones that lead to the power and reset switches and LEDs) are consolidated into one 20 pin ribbon. not only will that ribbon not fit onto any other mobo's front panel pin array, but the ribbon isnt even long enough to reach the front panel pin array on most other mobos...which leaves you with the dilemma of gettinga new case. i'll list the essentials to my upgrade (assuming a new P3 CPU is in order) since we basically had the same Dell PCs:
1) new mobo
2) new case
i got an ASUS CUSL2 mobo to run the P3 700E i got. i also got 2 sticks of 128 MB PC133 rev3 SDRAM from Mushkin. you can move the RAM from your Dell into the new system, but beware its only PC100. the Award BIOS that comes w/ the CUSL2 allows the memory to run @ 3/4 FSB though. i'm not sure about other BIOS's, so i would make sure which mobos support such options...otherwise i would upgrade to PC133 SDRAM. besides the Voodoo5 550 AGP (which i got well be my decision to upgrade), a new CPU, mobo, and RAM were the three things i got.
so dont think that you can skimp on any of these things while upgrading. a new case is a must unless you get another intel mobo, b/c the front panel connectors of Dell cases (w/ the 20 pin ribbon cable) are unique only to intel mobos. it wont fit on another board. and i wouldnt get another intel mobo anyways b/c they are IMPOSSIBLE to OC. most other cases have separate cables for the power switch, reset switch, sleep switch, and all the LEDs that go along with them, so you wont have the compatibility problems that you would have with trying to put a new mobo into a Dell case.
i just used the other components (sound card, fax/modem, NIC, etc.) from my Dell to finish off my custom system. my upgraded system is now a P3 700E that OCs to 1050 mHz @ the default Vcore of 1.7v. remember that high quality RAM is in order if you plan to OC much. if you have anymore questions, dont hesitate to ask.