While the maximum RAM limit for 32-bit Windows 7 editions is 4GB, when it comes to the 64-bit editions, the amount of memory that the OS can address depends on which edition you are running.
Here are the upper RAM limits for the different editions of Windows 7:
Home Basic: 8GB
Home Premium: 16GB
These limits are similar to those for Vista editions, expect that Vista Enterprise and Vista Ultimate have had their upper limits raised from 128GB to 192GB.
The one to look out for there is the 16GB limit on Home Premium. If you’re building a Core i7 system then it’s quite easy (if you have the cash!) to exceed this limit. As long as you’re aware of the limit and plan your OS accordingly you’ll be OK.