Wow those 4GB ram sticks are expensive.
Right now the only reasonable affordable choices are:
Get an i7 mobo and max out at 12GB
Get a server mobo (AMD's server cpus and mobos seem to be cheaper) that can be had fairly affordably with up to 8 ram slots, though it's likely a dual socket mobo, but AMD's low end 45nm opterons are pretty cheap (8 cores and 16GB of ram)
Or you can wait for DDR3 to inevitably reach 4GB a stick at reasonable prices.