addendum to above... as I had the edit window open overnight and didn't see the other replies before posting...
lots of good advice from others above!
I see you are building your own case:
what are you considering for a power supply? I really like the befaco excalibus - relatively cheap and quiet (most of my diy cases use these) - it's also available DIY - simple but tedious build (lots of power headers with 16 pins each) I'd recommend 1 per 6u...
I'd build 6u cases rather than 9u or a 9u and a 6u - all my DIY cases are 9u and they're a bit unwieldy... personally I find 6u/104hp as the best compromise... also it means that you can have 1 case vertical (or at an angle - 9u) and 1 horizontal in front (which I find easiest)
2600 is a good layout... I do something similar... sound sources at the top, modulation sources below that, then sound momdifiers, then control/sequencing/end of chain mixer at the bottom/front - utilities distrributed throughout
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities