I doubt you'll be able to upgrade it enough to handle Crysis...according to emachines, you're running a Celeron E1400, which will be a big bottleneck right there. Especially with Crysis being CPU intensive.
Honestly, your best bet may just be to save up for a new computer.
What video card did you put in there?
EDIT: Original specs:
eMachines Official Site - - ET1641-02w Product Details