The fans are fine, but you will definitely want to get an after-market cooler to overclock that up to 4ghz. Xigmatek is the new one that is recommended. It's not all that expensive either.
I know that the GPU will be hot, but the fans in that case move a lot of air. If you like the 900, you may want to check out the Antec Twelve Hundred - it's a larger, slightly improved 900 - full tower, more room/more fans, better cable management, and fan filters (less dust).
I am not sure about the PSU - you may want to google the power supply calculator and figure it out. I know those GPUs are power hungry and you will need some serious power on that rail.
im building a new pc soon, here are the parts i plan on buying
case: antec 900
cpu: e8400 (im gonna overclock it to 4GHz)
RAM: corsair 4gb ddr2 1066mhz
motherboard: Gigabyte GA-EP45-DS3
moniter: ASUS VW222U
video card: hd4870x2
i just have a few questions
will i need water cooling for a system like this? or are the fans in the case enough? You can probably do it on air but it's going to sound like a jet.
what psu should i get? is 650w enough? A good 600+ should do
also, how long will this last me until i have to do another upgrade?
??? With that monitor a very very long time, a 4850 or 4870 would be a wiser choice for you
how long? 2 years the most.... you dont need to run a water cooling system unless you want to spend all that money for it and you should buy other fans for the antec 900(the ones you can regulate its speed) and buy a control panel for the fans oh yeah and by the way it would be safer to choose a more powerful PSU