god, 640x480 is so yesterday >.>.
hit up the standard 1024x768 imo, also when designing a game its useful to work everything out before hand and make either a document,wiki, or notebook containing every detail about the game.
you dont want to "play it by ear." i know thats a bit OT but i think its important to be aware of.
As for the Character, it is very small but, small can be a great way to learn with pixel art. Animation is waaaaay easier the smaller it is.
The sprite looks very flat, the boots and body mainly Although i must say
for a first attempt its pretty good =P. Adding a light source from the center would make it more 3Dish and would eliminate any lighting issues in the game as Eyecraft suggested.
also i dont know what program your using, but if you want a pretty good pixel art program check out:
http://www.humanbalance.net/gale/us/ Although not as professional as say photoshop
its simplicity and design make for a pretty sweet combo. Good luck =) hope that helps at least a little bit.