Welcome to the forums!

If I was starting from the line I would make sure I was happy with the speakers and sub, and go backwards up the signal chain from there.

I'm still happy with my original HT speakers (over 10 yrs), and I've upgraded every other component since. Since I made the move to separates, I'd put the amp in as a close second, only because I'm confident I won't need additional channels any time soon. These days, any pre/pro buy is going to be trumped by future models fitted with HDMI 1.3 applications, so going with a 970 vs 990 would be a reasonable tradeoff if the money gets tight.

Hope this helps, I hope you enjoy the process of putting this together!