Default Best way to reclaim memory?

I've just bought a used HTC Desire, and after installing only 4 apps (Google Play Store, ES File Explorer, Screen Off and Lock and Swiftkey) it is reporting only 4Mb of free memory. What's the best way of reclaiming memory? Would it be a custom ROM? If so, can anyone recommend something that's very slimline and robust? I don't need the HTC bloatware (including Sense) - just Google Maps, Swiftkey and a few other apps.

The phone is currently running Android 2.2.2, Baseband version 32.48.00.32U_5.11.05.14

Is rooting/ROMing this device pretty straightforward?