All grain brewing is like an infectious virus caught by spending too much time on homebrew forums.
Yeah, I'll just add my two bobs worth here too. I built a three tier which works greatand I'm very happy with it. It keeps things nice and simple just using gravity and there are fewer things for you to worry about. Having been using my three tier for about 5 months now I can see now how much I would like to lower it down to a two tier or even one tier but at the moment that is of a minor concern. I will eventually do it but who knows when. In my opinion (which kind of fits in with the others above), if you think you will end up going to a two tier then you probably will "BUT" the thing is you can set up a make shift three tier system easily and use it now, then later convert it if you want to when you have a bit of extra cash or spare time down the track.
What I mean is, set up a temporary three tier (using everything and anything you have lying around, it doesn't have to look pretty eg. desks, chairs, steps etc. or lifting things) and start brewing. Later you might like to make a more permanent three tier if you like the system, or you can then create a two tier. All the equipment from a three tier will immediately and easily convert over to a two tier and chances are this time you will know what you want out of your system. Start simple and build on that. You'll be surprised what unnessesary things you'll think were important that you later find out make bugger all difference in the end.
Plus with a couple of brews under your belt from using your make shift three tier you will then gain the experience to know what's important so you can then build this into your next system.
Cheers and good luck.
Justin