Well you have two options:
1- Resell your current RAM and get this:
RAM: G.Skill 4GB 1600 DDR3
$114.99 with free shipping.
For mobo you have three options:
MOBO 1: ASUS M4A77TD
$74.99 after rebate and free shipping
MOBO 2: ASUS M4A785TD-V EVO
$99.99 free shipping
MOBO 3: ASUS M4A79XTD EVO
$109.99 after rebate and free shipping.
Option 1, RAM and mobo 1: $189.98
Option 2, RAM and mobo 2: $214.98
Option 3, RAM and mobo 3: $224.98
2- Keep your DDR2 and get one of this:ASUS M4A77DGIGABYTE GA-MA770-UD3
Now, choose the best option for you.