I’d go P4 – more stability (and more OC overhead) than the Athlon XP. Plus they cost about the same and nVidia and ATi both better optimized their drivers for Pentium 4’s.
And I’d take the Radeon 9000 Pro. You’ll hit higher 3dmarks with the nVidia Ti 4200, but the extra RAM of the Radeon 9000 Pro will make it faster further down the road. In addition, ATi’s drivers have improved greatly – just don’t get it on a Via chipset and you’ll be fine (Intel for an Intel board and SiS or AMD is what I’d recomend for an AMD board).