Upgrade Advice Needed

Hi!

First of all this is my current system
Amd X2 5600+ 2.8Ghz
Nvidia Geforce 7800GT XFX (Factory Overclocked + Zalman Fan)
KN9-SLI 570 Nforce
Hiper 480W HPU-4M480-EU
2048 Mb 6400 Corsair C5 DDR2 800

I'm thinking of upgrading to a

XFX GeForce 8800GTS 600M 640MB XXX

Now do i need a new PSU or not? Btw my Budget is Around 500$ May get a bit higher.
And may i have my zalman fan on it? it's model is : VF900-

Thanks in advance!