CIS for 1GB RAM 1.6GHz netbook

http://www.asus.com/Notebooks_Ultrabooks/Eee_PC_1001HA_Seashell/specifications/
It has Windows XP. The CPU is single-core but two-threaded, IIRC.
After I purged all the crapware and unneeded services I get 480MB (of 1024MB) of free physical RAM reported at startup. Previously it was 350MB.

Is it even reasonable to expect it to work at acceptable performance penalty? Let’s say 20% higher performance penalty that of Microsoft Security Essentials + Firewall is the maximum acceptable state.

If yes:
Please recommend an appropriate changes to the default CIS configuration.

If no:
Please recommend a free lightweight security suite as a supplement to Windows Firewall or as a complete replacement of MS software.

PS. Sometimes I deploy system installs on people’s netbooks. What would you guys say is the minimum specification for RAM and CPU for comfortable work with CIS installed, assuming 5400 RPM HDD?

I’d appreciate any help.

bump