What's an NCU and why is it important?
NCU basically decides how many buffs can you have. The more you have, the more beneficial nanos can be cast on you at the same time. Unfortunately, negative nanos can be cast on you even if you have your NCU space filled.
Each character has a default of 8 NCUs available when created. By equipping certain items like weapons, rings, armor, but mainly belts and memory chips you can greatly increase space for nanos. You can check what you have equipped in Weapons window, located by clicking the Wear icon (or CTRL+1). The most important skill needed for equipping any item involving those Deck slots is Computer literacy.
Since even the best memory chips have limited capability, you will want to equip as many of them as possible. For that you need a Belt. There are several of them available, mostly giving you the option to equip more chips, but some have a few nice modifiers of their own.
It's maybe one of the fields which were most influenced by SL and AI expansion. To see the NCU Belts and Memories that you can get from the Classic AO version of the game check here and if you're considering getting the Shadowlands expansion you may want to see what items you can get here.
Gives 6 NCU slots.
Adds lots of skills, most importantly 35 to Agility, Intelligence, Sense and Psychic.
Obtainable via tradeskill process involving parts from Sector 28
Can be further upgraded with a Green Xan Belt Tuning Device if you have the Legacy of the Xan booster pack.
Whatever expansion you have (or lack of it), the more memory chips you can equip, the better. Additional stats a lower belt might have, get overcompensated with buffs quite easily. For memory chips there's similar criteria. The more NCU space they add, the better. Possible exception might be specific chips with resistance to Snares and Roots, which are useful for PvP. Chips will always equip in first Deck they were designed for, so for adding more than one you need to drag it in proper place in Wear window.