Need help with uprade

[s1lent]

I don't have the most experience in PC building. What can I do to upgrade this PC to make it run like a dream? I love playing FPS online and I want to have a fast PC. My MOBO is only compatible with a few top of line CPU's. I want to know if I should upgrade my MOBO? Also I just purchased this GPU so would it be a good idea to run two of them and do I have enough power to run both SLI? I only have about $1500 to upgrade my PC. So what would be the first things I should do to get my PC running up to date?

What I am running right now.

Tower--Ninja (478x260x540mm)
ASUS Crosshair 940 pin motherboard (Nvidia chipset board)
AMD Athlon 64
x2 dual core 5600+ 2.91 ghz
XfX Geforce 9800 GTX + 512 DDr3
ValueSelect---4 gb
372 gb harddrive
Rocketfish 550w power supply
Vista 32bit


Any input would be greatly appreciated.

thank you

 

[Proximon]

You'll need to replace most everything I'm afraid. You can keep the HD and upgrade the video card later if you like.

What monitor are you using?

The good news is you don't have to spend all of that.
http://www.tomshardware.com/forum/261222-31-build-advice

Just give us a bit more info and we'll be able to help more.