Steinberg have released a statement explaining why Cubase doesn’t make full use of systems with more than 4 cores…
“This is why systems with more than four cores do not achieve the expected performance increase, especially at low latencies.”
…and that hasn’t changed much since Cubase 5. The same advise is also applicable to 6 core systems, it’s just when Steinberg released that statement 6 cores systems were not available yet. Dual 4 core systems were available before 6 core systems.
So if I were you I’d get the fastest 4 core system you can afford.
Regarding plectrumboy’s comment, maybe the 5820 does overclock well, but there’s no point getting it and overclocking it if Cubase doesn’t make full use of it.