The System Partition is the partition that contains the startup files, NTDetect.com, NTLDR, Boot.ini, and possibly Ntbootdd.sys.
The Boot Partition contains the system files (%SystemRoot%\System32).
Both can be on the same or different physical partitions. There can be multiple Boot Partitions, but only one System Partition.
The NTFS file system can address 16 exabytes (2**64), but current partitioning schemes limit a partition to 2 terabytes (2**41).
When a Windows NT 4.0 X86 based computer boots, it uses a INT13 function in the computers BIOS to gain access to the System Partition. This is a 24 bit function which translates to a maximum of 256 heads, 1024 cylinders, and 63 sectors of 512 bytes. If you do the math, the result is 8,455,716,864 bytes, or approximately 7.8 GB.
Setup is actually limited to creating a 4Gig System Partition because it starts as a FAT16 partition (unless you use an unattended setup and use ExtendOEMPartition). You can also pre-format the partition as NTFS and use the 7.8GB maximum.
The INT13 standard has been updated, so newer operating systems (Win95OSR2, Win98, Win2000) do not have these same limits.