Hi and welcome to ExTS
Confirm this for us please as it makes a difference re: RAM ( see below )
Start > right click on My Computer > Properties
It should tell you here.
If you have a 32 bit system you are limited to 4GB RAM - no matter what the motherboard capability.
The m/board will support 8GB DDR2 RAM
You can add 2GB ( 32 bit system ) or 6GB ( 64 bit system )
You don't say what PSU you have.
A decent Video Card is going to need around 650 Watts - possibly more depending on choice.
I suspect that the 7950GT only needs 400 - 450 Watts.
If this is the case you will need to upgrade the PSU too.
Far Cry 3 System Requirements ( recommended )
Intel Core i3 2.9GHz
4 GB RAM
Win7
DX11
12 GB free HD space
XP supports DX 9
There is an option here for a version of DX 10
click here
Your system could certainly be upgraded but with limitations.
What would limit it would be the PLGA775 socket and the CPUs it could run.
If 32 bit - 4 GB RAM max.
One of our other Mods may be able to add some more :)
EDIT:
Sorry Vr5fx I was writing this as you posted :)