RHEL3 => RHEL 5 - failover/clustering?

I just got 2 new identical systems to run our web servers.
They will replace our existing 2 systems running RHEL 3.

The 2 new systems are identical:
Processor: Quad 3.4GHZ/800MHZ/16MB Cache Dual-Core Intel Xeon 7140M Processor
Memory: 32GB DDR2 400MHZ Raid Enable, Dual Ranked DIMMs,
Hard Drive: 300GB, Gbps, SAS, 3.5 Inch 15K RPM hard Drive, Hot Plug


1) Is it recommended to install RHEL 5 on the new servers or stick with RHEL 3?

2) I have support for RHEL3, do I need to pay extra for RHEL 5 support?
   (the 2 old systems will be going offline as soon as the new one are ready)

3) Can I just download RHEL 5 and install it on my new systems.

4) Does RH have something similar to Heartbeat and drbd?

I was wondering if I can get ideas or can point me to where I can read 
on how to make sure that the systems are mirrored and are maintained
in synch and ready for fallback.

Any ideas are welcome.

Thanks,

Néstor :-)