As a rule off thumb.,,,,,
Windows7 is very simalar to windows vista in many ways but 7 has been optimised to run on lower power machines such as net books and laptops. However the end result (as far as gamers are concerned) is that this makes win7 an ideal gaming platform as it free's up more system memory for your games!!! win win!!
As such I'd go with the system spec for vista but take away 500mg from the memory spec.
E.g. crysis warhead....
Vista spec - cpu 3.2GHz, Mem 1.5 gig, Hd 2gig, vid card 256mb.
Windows7 - cpu 3.2GHz, Mem 1 gig, Hd 2gig, vid card 256mb.
However we all know system spec's are misleading and are more like bare minemum spec. If you want to play crysis in it's full glory in directx 10 with a minimum Frame rate of 25 fps+ on a 1920 by 1200 screen you will need.......
Cpu, core 2 @ at least 2.8GHz. Mem 4gig 800mhz ddr2. Vid card Nvidea 280gtx or radeon hd4870x2. Hd nice fast sata2 disk with plenty free space and you will need a 64bit os either vist64 or win7 64bit. allthough i have found win7 slightly faster (probably due to it's lower memory footprint).
@ highwayman.
Your idea about doubling the spec is misleading, i.e if you double the min vga card rating in crysis it would give you a 500mg card wich is still not good enough at high rezolution. really you need a 1gig card for todays high end games esp at high rezolution's and AA.