Right now I have a Mac Pro 2009, 8-core 2.26Ghz.
This machines almost equals the Geekbench score of the new 2013 quad core model.
Afaik Lightroom and Aperture only use CPU power, so here is the question: Will there be a big performance difference between the two models?
When I got my 6-core nMP around this time last year, I compared it to my 2009 Mac Pro Quad Core at photo tasks among other things...
https://forums.macrumors.com/threads/1692536/
Also, I wouldn't rule out Apple's Photo app. Based on the screen shots they showed at the announcement, it looks just as capable as Aperture. I wouldn't switch until you've at least seen reviews of Photos.