From a graphics/horsepower perspective, the Original Xbox was the most powerful console of it's generation and was giving the console gaming world a small taste of programmable pixel shading that would become defacto during the 7th gen.
Often I hear that the OG Xbox is the "Spiritual Successor" to Dreamcast, but find that to be pretty far removed, they are IP's that are owned by completely different companies even to this day.
From a hardware perspective...
Xbox used: Intel x86 processor, nVidia Geforce 3 Hybrid GPU, Mechanical HDD, 64MB of RAM.
Dreamcast used: Hitachi MIPS CPU, PowerVR, 26MB of various memory pools of RAM.
Completely different.
The OS side gets a little more interesting, Dreamcast used Windows CE Kernel, but most games actually didn't use it, they relied on Sega's software stacks instead due to performance/feature reasons, however Windows CE is based on a fork of the NT Kernel... But the underlying OS principles are very different to the Xbox Operating Systems. - Windows CE is a very modular/compartmentalized operating system.
On the OG Xbox Microsoft customized the NT5 (Aka. Windows 2000) kernel for the OG Xbox, they essentially took the NT5 kernel and stripped it as far back as possible to make it as lean and efficient as possible, it doesn't even have a registry for example.
It would be no different than saying the Playstation 4 is the spiritual successor to an Apple Mac or an Android Phone because despite the hardware sets being chalk and cheese, they are all derived from a fork of *Nix.
In regards to games, that is entirely down to personal taste, I haven't played many of Sega's games so I can't give critique there, but I did enjoy Fable and Halo o the OG Xbox back in the day.
--::{PC Gaming Master Race}::--
