Rincewind
I might be too late here, but I think you should play the previous Ultima games before Underworld 1. Not for story reasons (although knowing the Avatar's status in previous games does add to his predicament in UW1 to some measure), but for gameplay reasons. For me, having knowledge of Ultima 5's rune spellcasting system made me enjoy Underworld a lot more, because it uses the exact same system, just recontextualized. If you remember spells from U5, you can use them right away in UW1 even if they're not listed in the manual's spellbook, before you find the required incantation written in the game itself. If you remember the meaning of each rune character, then you can even deduce how to cast new spells. The same applies to the mantras; UW1 requires you to type in mantras at shrines to level up, and a lot of the mantras are the same ones you use at shrines from U4 onwards; so if you remember the meaning of each mantra and how it ties in to each virtue, and how those in turn tie in to your PC's attributes, then you can use that knowledge to your advantage.
None of this is really necessary; the manual explains most of it, and you're not gonna miss out on any spells or anything, because all that stuff can be found in-game through hidden notes. But for me, taking my knowledge from previous Ultima games to Ultima Underworld created a really awesome sense of continuity. In fact, gameplay continuity is what Ultima as a series does so well in every entry after U4. If you're really a stickler about playing Underworld before System Shock, then just go ahead; System Shock is too good of a game to be left waiting while you complete 8 other games. But in my opinion, you should play all the Ultima games in order, and play System Shock separately as its own thing, rather than seeing it strictly as a successor to Ultima Underworld.
But yeah, most likely I'll do the strict chronological route through the Ultimas then, and treat SS as a separate game.
