NetBSD 4.0.1 comes with XFree86, while NetBSD 5.0_RC1 will ship with X.org.
If you don't need 3D acceleration and you are working with 4.0.1, then I don't see why you'd need to build X at all. Just install X in the normal installation. The generic 'ati' driver should work just fine.
If you are building 5.0_RC1, then you can build X.org when you build world. Download xsrc from CVS and add these lines to your /etc/mk.conf:
Code:
MKX11=yes
X11FLAVOUR=Xorg
X11_TYPE=modular