|Ah you got one of those things :\|
Well to awnser for jam, when you power up your computer you have to get into the BIOS; most motherboards have this set on the small delete above your arrow keys, but some also have an Fkey assigned so reboot and look at the screen it should say along the bottom what to press to enter the BIOS.
I would assume however that the manufacture would have set this to the mapped memory size of the graphics card onboard, if not your looking for 'Video Ram Size' or simularly named option. Use your arrow keys to get around in the BIOS and find this; then set it to 128mb. Since you have 756mb system ram; thats provding the BIOS even will let you adjust it - these crappy emachines ( no offense ) sometimes have a passworded BIOS too.
WinXP PRO SP2
BIOS 1014 beta 2
PC3200 768 1.5t 3t 3t 6t
AMD XP 2600+ @ 2.2
ATI AIW 9800 PRO 128MB