Hi Aman,

From 4.2 there is a direct upgrade path to 6.x which is very good;

Cisco CallManager Upgrade Paths

http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_tech_note09186a008086cc7c.shtml

System Requirements

Make sure that you install and configure Cisco Unified Communications Manager Release 6.0(1x) on a Cisco Media Convergence Server (MCS).

You may also install Cisco Unified Communications Manager on a Cisco-approved HP server configuration or a Cisco-approved IBM server configuration.

Cisco Unified Communications Manager requires a minimum of the following items on the Cisco MCS.

2 GB of memory

72 GB disk drive

2 GHz processor

Supported Platforms

To find which servers support the Cisco Unified Communications Manager Release 6.0(1x), refer to the Cisco Unified Communications Manager Server Support Matrix at;

http://www.cisco.com/en/US/partner/products/hw/voiceapp/ps378/prod_brochure_list.html

and

http://www.cisco.com/en/US/customer/products/hw/voiceapp/ps378/prod_brochure0900aecd8062a4f9.html

From these CCM 6.0.(1a) Release notes

http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/rel_notes/6_0_1/601acmrn_2.html#wp42710

Hope this helps!

Rob