I just ordered a 8800GT for my new rig which I don't have yet. But I was wondering if it will run OK in my old system for now?:
 
AMD Athlon 64 2.2ghz 3500+
Asus A8N-SLI Deluxe
3gb DDR 400
Enermax 12V Ver2.0 485W Power Supply

You'll obviously need a PCI-E slot, which your mobo seems to have.  You have sufficient power.
 
Why else might it not work?

I just put one in with my Opty 170 on an MSI nforce4 ultra board and it works wonderful

Your CPU will hold you back a bit since it's a single core, but it'll still work.

It should work fine. However, it might instill in you the desire to upgrade your entire system. That's what happened to me when I plugged my shiny new 8800GT into my well aged S939 board

of course it will... if you have a PCI-E 16x slot on your mobo. It doesn't matter how slow your computer is (unless of course it's not fast enough to run the drivers )

turns out that the damn thing wont fit in there anyways becuase the bulky after-market heatsink on the southbridge chip is in the way!
damnit

Yikes, did you look that up or find out the hard way?

i tried to put it in
 
apparently the south chip on that board gets really hot and it's common to put extra cooling on it

Come to think of it, there was barely enough room on my old MSI K8N Neo4-F for my 8800GT. The card came like a millimeter from the northbridge fan; made me kinda nervous.