How much memory does the os and win7 use?

OS X 10.8 Mountain Lion will take around 5 GB, iLife will take another 3 to 4 GB, your personal data might take from 100 MB to several dozens of GBs depending on your usage.
Windows 7 takes less than 10 GB away.

Take at least the 128 GB SSD version, if not the 256 GB SSD if your budget allows this.

Btw, "memory" is not the same as "hard drive / storage capacity":

• Computer Storage: Ram vs. Hard Drive Space
• RAM vs. Hard Disk

It is recommended, to have 10 to 20 % of the SSD/HDD's capacity unused.
The above links also apply to SSDs.

My MBA had around 100 GB free out of 128 when I got it with ML preinstalled.

I have a clean 32-bit W7 installed in a VM and with all latest W7 updates it occupies close to 15 GB. The 64-bit W7 version probably takes a bit more (perhaps close to 20 GB). Installing with bootcamp instead of a VM may also require a bit more space (not 100% sure about that).