More importantly, you need to figure out what Cubase 5 can actually use.
-Is it 64bit? If not you won't see any advantages using w7 etc, apart from general stability etc.
-Does Cubase use multiple cpu cores? If so, the more cores the better. If not, any highly clocked dual core is fine.
-Have you checked your ram usage? Do you actually need more?
Problem is we know very little about your software so can't really say what hardware you will need.
If you are smart enough to pay for the software you can always contact them and ask what works best.