One thing, you said that your case would not fit a big card like the 8800GTX. I would strongly suggest you get a bigger case then because it is likely that the R600 will be a big card as well. Also, you are gonna want some room in there to make sure your components stay nice and cool. There are cool looking full tower cases too.
And on the GPU issue, if you don't want to get a DX10 card until the R600, thats fine, but I would suggest getting a cheaper DX9 card so you will have enough money to buy the R600 when that time comes.
Oh, and btw, nVidia is not "meh", I have an 8800GTX and it is frickin amazing. Runs incredibly fast and everything looks beautiful. nVidia is amazing. Don't take this as a bashing on ATI either, I did not say one word about ATI.