I say go #1, and see how much of a hit your performance takes.
If acceptable with 2Gb, save up money and build new system.
If unacceptable, buy new RAM and then save up money and build new system.
If drunk, build new system. (Then have another beer)
The obvious answer is to start with option #2, and then #3 when you have more money. It's not like you can't just upgrade one or two things at a time.
Thing is, I guess the new system that Chew is envisioning will likely not be based off the same types of components. All the cool kids use intel i7's nowadays, and even if sticking with AMD it may be a different socket. I have no idea what sockets AMD's use.
Plus I'd imagine Chew wants DDR3, which wouldn't be supported by this mobo I'm guessing its DDR2 by price and age.