Unless they buggered the xbox version, which i cant see how, i doubt the graphics would be "better" on a pc. They should look the same. From the demo, the game looked similar to other steam games, so if you've seen them running, then think of that.
L4D isn't the next Crysis or ( insert console equivalent ), and it was never trying to be. It is however a pretty fun game to play. Though, I never thought the fire was horrible or the character models crap.
I also can't see the graphics being worse on the demo, it's not a beta, though i suppose it is possible they made some last minute adjustments, but i doubt they'd b major. If you think CSS and TF2 on a pc look crap, then I think you'll think the same about L4D