Registered: Apr 2010
Location: Continental USA
Distribution: Debian, Ubuntu, Fedora, RedHat, DSL, Puppy, CentOS, Knoppix, Mint-DE, Sparky, Vsido, tinycore, Q4OS
You CAN install Win7 later, but it is not as straightforward. You will need to install (or re-install) grub to the MBR from a live-cd after installing Win7, because MS install assumes that only MS products do or can exist and will overwrite it.
A better option, if you have the space and horsepower, is to install virtualization and run Win7 in a virtual guest under Linux. Avoids all the hassles, and you get to run both desktops at one. SEE them at the same time, if you have dual monitors.
I run the other way: WinXP as a base, with virtual guests of Win7 and Debian, but only because the company demands I run Windows on my laptop.