The processors arrived today and I've been doing stability tests on the R510 since around noon. I chose the 6-core E5645 2.4 GHz, which are fairly expensive. There are a ton of ES ones available, but I wouldn't trust those in a "production" system. Normal price has been $500 each, but I snagged these non-ES versions for $429 each.
Compared to my dual socket 8-core Opteron server (16 cores total), one of these servers is roughly 25% faster. Considering that there are fewer cores, that is pretty impressive. If you
go back a few pages, you can see that during stability testing, the Opteron setup did 77 GFlops. I had to run two instances on this server to full load all the cores, so that probably degraded the performance slightly. It is doing a combined 96.53 GFlops (25.3% faster). I'll do another run with only a single test running to see how it does when there is no competition for memory.