Can I upgrade the IOS from 6.3 to 7.2.What will be the impact,Will the existing config will be erased or will not be erased?
Yes you can upgrade from 6.3 to 7.2, go over this link for requirements, config will not be erased, however there will be automatic conversion from 6.3 commands such as conduits statements etc.. convertion to 7.x code.. see changed and deprecated features section.
Upgrade notes details
http://www.cisco.com/en/US/docs/security/asa/asa70/pix_upgrade/upgrade/guide/pixupgrd.html
Memory requirements details for PIX platforms before upgrading to 7.x
http://www.cisco.com/en/US/prod/collateral/vpndevc/ps5708/ps5709/ps2030/prod_bulletin0900aecd8023c8d4.html
Can I able to creat the TRUNK between a PIX and Cisco L2 switch?
Yes, you should be able to use 802.1q , the syntax commands are different from that of 6.x code, in code 7.x above , you will use subinterfaces to split single physical port to several logical interface.
http://www.cisco.com/en/US/docs/security/asa/asa72/configuration/guide/intrface.html
And also I need to configure the PIXs in FO, that is one in active and other in passive,in that case both FW should be enabled with the necessary license?
same license principle applies from 6.x. At least one unit must be UR , the other FO for active/standby, see license details
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_tech_note09186a00806b1c0f.shtml
Standby/Active config
http://www.cisco.com/en/US/products/hw/vpndevc/ps2030/products_configuration_example09186a00807dac5f.shtml
Rgds
Jorge
Jorge Rodriguez
