I cant decide. I have a Amd 64X2 5600+ and Asus M2n32 SLI mobo and centurion 5 case. My pc is great. I was thinking, should I get an Antec 900 and a new CPU cooler or should I keep my case and buy a EVGA 680i and use the parts I have now until I get a intel processor??? Or, I could keep my rig and get an AMD 6400+ or maybe Phenom

If its great how it is now why upgrade?
 
If it for gaming you wont see a huge increase in performance from a  relatively cheap intel CPU. Personally i like the centurion 5 over the Antec 900, but if you really want a new case why not look at the coolermaster  CM 690.

That rig is a great gamer. I would not spend a penny on it atm. Wait a few monthes. When the next line of AMD releases you should have some new Intel as well. Make your move then. There is nothing worth buying that will improve your gaming atm unless you want to spend insane amounts of money to mount a new Intell and OC the hell out of it and then replace the cpu in 2 years for a quad anyways. Enjoy your up to date rig as it is.
 
You cant realy compare the Antec 900 to the CM. One is super budget the other isnt. The Antec has much better cooling options and the side cover fan doesnt hit your zalman cooler.

I agree, I don't think I would change anything. If you want to push a little more out of that system, you could OC the 5600+ and get a little boost.

I agree with Jersey about the cases as well. Not a whole lot to compare between the 900 and Centurion 5.

@Jerseygamer, your name implies that you are from New Jersey (unless I am being stupid, which is entirely possible). What part are you from if you don't mind me asking?