What CPU heatsink for AMD 955 in antec 900 case

I'll point you here first. Just about every cooler will fit in that case, you might not be able to use the side fan, or might have to cut a corner off a fan. You also need to watch out for the height of your memory. If they have those crazy high heatsinks on them some coolers block the first few mem slots.

But look here first, choose your best options and then look into total height and mem issues.
http://www.frostytech.com/top5heatsinks.cfm

These issues have been discussed a lot here, so you might want to do some mind numbing post searches and manual digging. You might find your answer..

With a set of 2x 2Gb of ram you will not have any issues as far as clearance goes regardless of the CPU cooler. The issue is when you have all 4 banks occupied.

How much do you want to spend on the CPU cooler?

^5 +1 what Conumdrum said.

Frosty tech is a very good web site that has a lot of useful information about cpu heatsinks. It can help you make an informed decision.