Upgrade ZX Spectrum 48k with 128k of RAM and play all the games that you could not play before. Only the sound will stay as before, but you can always buy an extension module for sound (based on AY-3-8912 or AY-3-8910) to make it sound as ZX Spectrum 128k.
In order to make that extension card work, a small modification to your ZX Spectrum 48k is required. There are two options for that:
Option 1: remove all the upper memory ICs (IC15, 16, 17, 18, 19, 20, 21, 22).
Option 2: on IC23 bend the pin 1 and connect it to pin 14 (+5V).
With the mod from Option 2 but without the External RAM Extension, your ZX Spectrum 48k will still work but as ZX Spectrum 16k.