Asus A8N-SLI - best components?

Hi,

I'm looking to boost the performance of my computer by getting a new processor and adding a second graphics card - but am not precisely sure what to do.
(Basically, I got Call of Duty 4 - it looks shocking on my PC and I want to see if I can get a bit more power out of my system without starting from scratch!!)

I have a Asus A8N-SLI motherboard with one Radeon X1600 Series graphics card.

The processor is a AMD Athlon 64 X2 Dual Core 3800+ (Socket 939) and I have 3GB of RAM.

My questions are as follows:

1. Can I add another identical X1600 graphics card to increase graphics performance?
2. If not, what would be the best upgrade for graphics cards?
3. What would be a good upgrade for my processor?

NB. I did notice on the Asus site a comment regarding the motherboard:
'Under SLI mode : support two identical SLI-ready graphics cards'

But wasn't sure if that actually answered my first question (I'm not too crash hot on hardware!!)

Cheers guys!