I'm going to upgrade my CPU soon, and was thinking of getting the AMD 955. However, I have read that older AM2+ motherboards might not support L3 cache for CPUs. How can I tell if this is the case with my motherboard? It is an ASUS M3N78 Pro:

http://www.asus.com/Product.aspx?P_ID=DVvm9CU0G1bCC4gp


If my motherboard can not support L3 cache on the CPUs, am I better off upgrading to an AMD Phenom II X4 840 over the 955? It uses less power and seems to be just as fast aside from the lack of L3 cache, but costs a lot less.

From the link you posted, "This motherboard supports AMDŽ Socket AM2+ multi-core processors with unique L3 cache." So, I presume the same goes for L3 on an AM3 CPU. You should be okay.

I'm not sure when Bulldozer is expected to hit, but if you're committed to AMD you may want to wait until then; surely, its release will drive down the prices of the existing Phenom II stock.