Don't forget IPS screen that you can get off ebay for $50! The biggest upgrade I've done to my x220. I think you can even get FHD moded IPS somehow, but it looked like a bit of work last time I checked.
IPS is the easiest and biggest single improvement for the X220. Coreboot's the next single biggest improvement, although it's not easy.
I bought a used x230 recently. My only gripe has been the washed out display - I should give this a go. Otherwise the performance is very good for a machine that was so inexpensive.
I'm going to be selling my x230 soon. I've got a mod chip and 1080p IPS display I could sell you.
I am waiting on the backlist for the FHD mod... I already have the IPS screen forgot to mention that!