Yavuz,
You will need the Advanced Security feature set.
Here is info on the most current IOS-
ADVANCED SECURITY
c7200-advsecurityk9-mz.150-1.M3.bin
Release Date: 19/Jul/2010
Size: 41229.44 KB (42218940 bytes)
Minimum Memory: DRAM:512 MB Flash:64 MB
You do not have enough DRAM/Flash to run this though.
The leatest IOS that will fit on your router is
SERVICE PROVIDER/SECURED SHELL 3DES
c7200-k4p-mz.120-33.S7.bin
Release Date: 02/Jul/2010
Size: 11038.04 KB (11302948 bytes)
Minimum Memory: DRAM:128 MB Flash:16 MB
You should double check this supports IPSec in the feature navigator.
Here's a link to configuration guides for IOS IPSec.
http://www.cisco.com/en/US/partner/tech/tk583/tk372/tech_configuration_examples_list.html
Hope it helps.