What are the maximum and minimum limits for RHEL 4 vs 5 vs 6 ?

Below chart shows you the comparison of various features of Red Hat Enterprise Linux between version 4, 5 and 6.

	Version 4	Version 5	Version 6
Maximum Logical CPUs
X86	32	32	32
Itanium 2	256/512	256/1024	NA
X86_64	64/64	160/255	160/4096
POWER	64/128	128/128	128

Maximum Memory
X86	64GB	16GB	16GB
Itanium 2	2TB	2TB	NA
X86_64	256GB/1TB	1TB	3TB/64TB
POWER	128GB/1TB	512GB/1TB	2TB

Required Minimum
X86	256MB	512MB minimum/1GB/logical CPU recommended	512MB minimum/1GB/logical CPU recommended
X86_64	256MB	512MB minimum/1GB/logical CPU recommended	512MB minimum/1GB/logical CPU recommended
Itanium 2	512MB	512MB minimum/1GB/logical CPU recommended	NA
POWER	512MB	1GB minimum/ 2GB recommended	2GB minimum/ 2GB required per install
Minimum Disk Space	800MB	1GB minimum/ 5GB recommended	1GB minimum/ 5GB recommended

File systems & storage limits
Maximum filesize (Ext3)	2TB	2TB	2TB
Maximum filesystem size (Ext3)	8TB	16TB	16TB
Maximum filesize (Ext4)	---	16TB	16TB
Maximum filesystem size (Ext4)	---	1EB	1EB
Maximum filesize (GFS)	16TB/8EB *	16TB/8EB *	NA
Maximum filesystem size (GFS)	16TB/8EB *	16TB/8EB *	NA
Maximum filesize (GFS2)	---	100TB	100TB
Maximum filesystem size (GFS2)	---	100TB	100TB
Maximum filesize (XFS)	---	100TB	100TB
Maximum filesystem size (XFS)	---	100TB	100TB

Kernel and OS features
Kernel foundation	Linux 2.6.9	Linux 2.6.18	Linux 2.6.32-2.6.34
Compiler	GCC 3.4	GCC 4.1	GCC 4.4

NOTE: * If there are any 32-bit machines in the cluster, the maximum gfs file system size is 16TB. If all machines in the cluster are 64-bit, the maximum size is 8EB

Reference:
Red Hat Enterprise Linux technology capabilities and limits