More is better. I use an iMac (2011) with 16gb RM and I've found that PPCC can drain the memory like crazy especially when using certain effects which is why I'll be getting the new 2013 iMac with the best GPU and 32gb RAM in the next week or so. We use Warp Stabilizer a ton (as in hundreds of times per project) so after only a couple of hours we've seen our "available memory" drop from say 12gb to like 15-20MB (yes, megabytes) and Premiere Pro CC starts doing some weird stuff. I end up restarting my computer several times a day. Really annoying. For normal basic cutting, it doesn't seem to use all that much memory so I'm sure 8gb is fine. You can always try it and if it's slower than you'd like go to macsales and bump up to get more.
[Moved to Hardware forum.]
With HD media you want 16GB of ram or more. 8GB is really to low with GPU acceleration.