Yes, I installed it on one yesterday.
Refer to table near end of this link excerpted below:
http://www.cisco.com/en/US/customer/products/sw/secursw/ps2120/products_data_sheet09186a0080148714.html
" Minimum RAM
32 megabytes (MB), except the Cisco PIX 501 Security Appliance, which requires 16 MB
Minimum Flash memory
16 MB, except the Cisco PIX 501, 506, and 506E Security Appliance models, which require 8 MB "
- John
