My XBox is currenly disassembled, so I can't tell you what's on the hard drive, but I can think up a list of some games I enjoyed (and some that might be good but I haven't played very long yet):
OutRun 2, OutRun 2006: Coast 2 Coast -- very similar games, both very awesome...I'd say it's worth having both of them if you like one of them, since all features are not in both games
Serious Sam - I know this is already on your list, but I just wanted to say, one of the few games where I think the console version is better than the original PC game. Too bad it has really bad screen tearing, though =(
Call of Cthulhu: Dark Corners of the Earth -- very awesome game, most modern horror games (Amnesia and etc.) are based on this. I liked the PC version better because of the mouse controls, but the console version might be less buggy, and it actually uses the force feedback as a game mechanic, which is really nice.
Final Fight: Streetwise -- I have no idea why this game gets bad reviews, I liked it a lot. The music can be turned off if it bothers you. The two-player mode is kind of dull, though.
Gauntlet: Dark Legacy -- it's Gauntlet Legends with remixed levels and bonus content!
Baldur's Gate: Dark Alliance, D&D Heroes, Justice League, Marvel Ultimate Alliance, etc... -- blatant Diablo II ripoffs that can be played with up to four players on one TV. There's a lot of these kinds of games on the XBox, and I enjoy most of them (Justice League gets old pretty fast, though).
Metal Slug 3, 4, and 5 -- much better packages than Metal Slug Anthology, lots of bonus content.
CoD: Big Red One -- I don't usually like CoD games, but this one is from the same devs that did Return to Castle Wolfenstein, and I had a good time with it (and it doesn't have the auto-regen health feature). CoD3, on the other hand, was really bad.
Panzer Dragoon Orta -- another Sega rail-shooter...or whatever that genre is called. Includes a port of the Sega Saturn original game, too.
Alien Hominid -- if you're looking to actually purchase these games, I think this one was only released in Europe (but I could be wrong).
Games I haven't played for very long:
Sonic Heroes -- another game with bad reviews that I did not hate.
Unreal Championship -- watered down version of UT 2003, but split-screen makes up for it!
Psychonauts -- I really need to get around to playing this. I also have it on PC, but will probably play it on console.
Jacked -- apparently some ex-devs from Road Rash worked on this...I will be playing this as soon as I get my XBox fixed.
Toejam & Earl III -- need to play this some day.
Shenmue II -- if I ever beat the first game, I will probably play the second one on XBox, because they had to cut a lot out to get the English translation to fit on Dreamcast CD-Rs.
Phantasy Star Online -- I haven't played the XBox version very much, but it has split-screen multiplayer and a few other new things that weren't in the original DC game(s).
The Thing -- supposed to be pretty cool, and I love the film, but I got stuck right at the beginning of the game and haven't bothered looking up a walkthrough yet.
Rayman 3 -- I was enjoying #2 on the DC for a while, so if I ever beat that, 3 is also supposed to be good.
SoulCalibur II -- if I ever got bored of the first game (which I probably never will) or had no way to play it, I would play this game instead.
I haven't listed any games that are superior on the PC, like Thief 3 or Arx Fatalis.
Also, I don't use cheat codes, and I don't know what your definition of "manageable difficulty" would be, but I couldn't think up a very big list, so you can weed them out yourself if you like. Right now, I can tell you, Panzer Dragoon is pretty difficult without cheating
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I'll brave the storm to come, for it surely looks like rain...