ThermalRight Ultra 120 Extreme and Xigmatec Dark Knight are the ones I am using in my 900's. So anything that size will fit.
BTW, the 900 does not come with a system (case) speaker. If your motherboard does not have one, you can buy one here: http://www.cwc-group.com/casp.html
The Antec Nine Hundred case is 8.1 inches (206mm) wide. It can accomodate the 160mm tall cpu heatsinks such as the Ultra 120 Extreme or Dark Knight that jsc mentioned.
Here is a link to a very good web site with a lot of useful information about cpu heatsinks that will help you make an informed decision:
I use the Xigmatech Dark Knight in my Antec900 on an i7920 and it fits just fine. It looks like it's not going to ... but it fits without hitting the side panel. Plus, since positioning of the very big 140mm exhaust fan on the top of the case coupled with the direction of the HS/fan when installed it makes for a decent cooling solution. I have my rigged overclocked to 3.82ghz with turbo on and HT enabled and my idle temps are 30C and about 50C under load after running prime.