At this point honestly, your pretty screwed.
There is a solution, but it's not a very good one. Here's the deal.
You wanna use your P4 478, but you need a decent videocard. 99.9% off all 478 mobos are AGP. At this point, you're not going to be able to get a decent AGP video card for less then $350. And for that money you could easily get a MUCH better PCI Express card.
So, how about that other .01% of 478 MOBO's with PCI Express?
Not exactly cream of the crop. But, hey it might do? Only it's not available anywhere... lol
So that leaves you to your last choice. Save your 400 and continue to save until you've got about 600 to 800 then pick up a AMD 3200+, DFI Lanparty UT, a 6800GT or 7800GT PCI express card and a new PSU. You're going to need a 24 pin PSU. Ah.. let's say 600 to 650?
Now you've got a gaming CPU and gaming video card, easily overclockable MOBO and enough juice to keep them all going...
? What do you think?