Well, with a budget of 800 MS Points, I see a few options.
The first would be getting the free download of the FX Pinball 2 base platform, and using those points on the Marvel Superheroes tabels. Pretty good value for the money, especially if you're a pinball fan. These are some pretty good tables.
Second, Castlevania: Symphony of the Night. This game is, in my not even approaching humble opinion, a masterpiece. This is, by far, the best thing Konami did in the 1990's. And, yes, that includes Metal Gear Solid. Metal Gear Solid sucks. There. I said it.
Third is Peggle. This game is just plain addictive. And, it's brilliant.
Fourth is Puzzle Quest. The best way to describe Puzzle Quest is to call it "Bejeweled with a definitive goal." If you're into puzzle games, grab the demo. I think you'd like it.
Fifth is the old school arcade route. Both X-Men and Teenage Mutant Ninja Turtles are on XBL, and the games are the same fun that they were 20 years ago. Granted, that's a little bit of nostalgia with that sentiment, but they are still worth playing.
The last option I can think of, and the one I would personally choose, is to go through the indie games, and grab a bunch. There really are some gems there. Case in point: The Impossible Game. It's damn hard, and I can't get enough, even though I fail at it every time.