The trailer is surprisingly good considering the Xbox is a Celeron 733 with 64MB of RAM and a Geforce3-level video chip.
It's still noticeable though. The resolution is like 640x480 without any AA and the textures are visibly lower resolution. There also appeared to be some low framerate parts, although those parts may have been slowed for dramatic affect. The PC has a much crisper high defenition feel to it. DOOM 3 would be a great launch title for XBox 2 though, but I guess that's a little too far off.