Every 3D game has their limitation in polygon count, normally the range for a low poly character is around 1000-5000, depends on how detail the character is going to be. but as the technology keep developing, game modeler can put more polygon on a character for the next genration like PS3 or Xbox 2.
512X512 is the standard Texture size for more games i think.
if you have time, go check out
www.cgtalk.com , there you can find lots of information regarding game art.