max2, You could quite easily upgrade the battery provided you could find one the right dimensions.
If your battery is 3.7v 200ma Lithium Polymer you could replace it with say a 3.7v 400ma battery which would double your battery life.
Regarding the player spec you mentioned
2GB or more
1.8 TFT screen
Long battery life - more than 4 hours
I just bought one
HERE
Will let you know if it lives up to it's specifications.