For my personal gaming rig, I'm getting a crossfire motherboard when they come out, but if I were to get an NF4 board, it'd be the DFI Ultra-D, or if i'd have been smart and gotten a 7800 GT, a SLI-DR, so I'd have an inexpensive upgrade option for gfx in a year or so. If you're willing to wait, the Abit crossfire board or the ASUS one look pretty good, supposed to be good overclockers, nice feature set, etc. As for the CPU, unless you have money to burn, don't get an FX chip. the 4000+ SD is an FX-53 rebadged, with about $500 off the original price. for a little less you could get a 3700+ SD and overclock it. If you want the absolute best, your money would be better spent getting the latest A64 X2 chip. FXes are a waste of money unless you want to overclock with upper multipliers, which is ineffecient because it's JUST the CPU, not the FSB, so the RAM stays slower. Unless, of course, you just want your son to have the bragging rights of "yeah, that's right. I got an FX chip. EAT THAT!" or something. Just some advice. Also, if you're dead set on an ATI card, I'd wait for a crossfire board, because in a few years when that X1800XT PE can't run Quake 6, you can get an X1800 XT master card and come close to doubling the GPU power for about $50-$100.
EDIT: to answer your original question, I'd say Nforce 4 Ultra/SLI or Crossfire (in a month or so)