I decided on 48GB for the Windows 7 RTM from Technet with MOST apps, no hibernation file and pagefile moved to another HDD.
Prior upgrades from 7100>7127>7232 had required ~20GB FREE space (some reported less). This may not be true for post RTM upgrades but WINSXS tends to grow, HDD space cheap and moving apps to recover space is a PIA.
I have a second partition for non-critical apps which are HDD space hogs (i.e. EMC10) and games. STEAM gets its own partition. These I can move around at will.