The original ZX Spectrum is remembered for its rubber keyboard, diminutive size and distinctive rainbow motif. It was originally released in 1982 with 16 KB of RAM for £125 Sterling or with 48 KB for £175; these prices were later reduced to £99 and £129 respectively. Owners of the 16 KB model could purchase an internal 32 KB RAM upgrade, which for early "Issue 1" machines consisted of a daughterboard. Later issue machines required the fitting of 8 dynamic RAM chips and a few TTL chips. Users could mail their 16K Spectrums to Sinclair to be upgraded to 48 KB versions. To reduce the price, the 32 KB extension used eight faulty 64 kilobit chips with only one half of their capacity working and/or available. External 32 KB RAM packs that mounted in the rear expansion slot were also available from third parties. Both machines had 16 KB of onboard ROM.
About 60,000 "Issue 1" ZX Spectrums were manufactured; they can be distinguished from later models by the colour of the keys (light grey for Issue 1, blue-grey for later models).
Source: Wikipedia, "Zx spectrum", available under the CC-BY-SA License.