Oracle Database Discussions

My Oracle instance is shutting down like anything. Have bounched it several times. I know this is not the solution, tried to change the spfile settings, but did not work.

Details: Have 8 GB RAM, Windows 32Bit/Oracle 10G R2.
Currently, the details about the parameters are as follows.

C:\Drive (20 Gb)
E:\Drive (300 Gb)
Oracle installed on E drive.


*.db_block_size=8192
*.db_file_multiblock_read_count=32
*.db_name='DEVREVDB'
*.db_recovery_file_dest='E:\oracle\product\10.2.0/flash_recovery_area'
*.db_recovery_file_dest_size=2147483648
*.dispatchers='(PROTOCOL=TCP) (SERVICE=DEVREVDBXDB)'
*.job_queue_processes=10
*.open_cursors=300
*.pga_aggregate_target=1048M
*.processes=500
*.remote_login_passwordfile='EXCLUSIVE'
*.sga_max_size=1221225472
*.sga_target=1221225472
*.shared_pool_reserved_size=12828800
*.shared_pool_size=128288000
*.sort_area_size=4192000
*.star_transformation_enabled='FALSE'
*.statistics_level='TYPICAL'
*.timed_statistics=TRUE
*.undo_management='AUTO'

Tried all possible ways to improve the settings. but could not get beyond.
If i can get to know as to what should the SGA_TARGET, SGA_MAX_SIZE, PGA_TARGET_SIZE and other parameters should be set to, it would be of great help.
The total RAM on my system is 8 GB.