12 cores is overkill right now unless you're doing massive scores with VI's or mixing film soundtracks natively. As long as you are running PTHD, your mixing is done TDM and more cards in an expansion chassis is money better spent than on native power.
I have an 8 core 2.8. these are the older early 2008 models. They came in an 8 core 3.2ghz as well. they were more powerful then the newer machines up until this newest batch of late 2010. Intel changed chip architectures to reduce power consumptions and heat etc and thus slowed the speeds down but increase processes per cycle. But all that doesn't really help unless software is optimized for taking advantage of hyper threading and turbo whatever they call it. My recommendation is to try and get a deal on used 2008 8core 2.8 or 3.2 if you can find one. And don't buy your ram from apple its ridiculously over priced. I bought 8 gigs of kingston for few hundred on Ebay. Would have been over a $1000.00 if pre-installed by apple.
Here is an excerpt from a review on computershopper.
"of course, aesthetics aside, the main thing the Mac Pro is known for is its performance. The model we tested is the low-end option, so we didn't expect ground-breaking results, but at $2,499, we didn't expect it to be a slouch either. And for the most part, this machine didn't disappoint. In Mac OS X, it turned in a score of 15,302 in our CPU-centric Cinebench 10 test. That's not as high a score as we saw in the dual-Xeon-CPU Mac Pro we saw back in 2008 (which scored 18,070 on the same test), but that decked-out older Mac Pro had a price tag close to $4,000 in our test configuration. In our 11-song iTunes conversion test, the newer Mac Pro came out solidly ahead, finishing in 2 minutes and 56 seconds, 18 seconds ahead of the 2008 Mac Pro. But the most recent iMac we looked at, the 21.5-inch 2010 model (powered by an Intel Core i3 CPU), finished our iTunes test just 4 seconds behind the Mac Pro here. As iTunes can only access a single processor core, this reiterated our insistence that you need to be running multi-core-aware software to take full advantage of the Mac Pro's Xeon CPU."
http://computershopper.com/desktops/reviews/apple-mac-pro-20 10-version