I recommend saving for 600, it would be really worth while. Right now the PC market is pretty good in terms of price. Lots of powerful parts for cheap because the current generation of consoles are still around like ancient dinosaurs. Most games are designed to work for those. If you can save for 600 it will get you much much further than 400 alone could and then buying parts sequentially. Also you may wish to check out NCIX.ca They can have some pretty decent deals too.
Do you think that'd work? I mean, in the end it would be over $600 since I'd be spending $400 on whatever, and then however much to improve it so it would be as good as a $600 computer would have been at the start.