two problems here
2GB is the absolute minimum to run Win7 - doubling it to 4GB would be the cheapest way of getting an upgrade. Taking it beyond 4GB won't make much further difference
the other problem is the CPU - most of these were fitted with Celerons. Nothing much you can do about that - the machine is essentially compromised as a result.
Only other thing you can really try is to optimise the software - uninstall / disable as much of the bundled Acer bloatware as possible, and use an antivirus program which doesn't have too much of a memory footprint. Disable things like Realplayer and Quicktime from loading at startup.
However its easy to get that kind of thing wrong and cripple the machine