A few words of advice from a fellow music producer:
1. Forget AMD CPUs In music production Intel CPUs are much more efficient at low latencies (with ASIO buffers @ 128 samples and below). A quad-core i5 760 should give you a better performance than a six core Phenom. (In the realms of DAWs 'performance' actually means how many VST/VSTi plugins you can use at once without freezing tracks and keeping the latency as low as possible). It seems the the sweet spot for price/performance is Core i7 860.
2. Have a look/join the forums @
www.dawbench.com This is the only website that I know of that tests various CPU/OS configurations for music production. You can download the test suite(s) and try it yourself
3. Leave some room in your budget for a good audio interface - think about stable drivers and low latency operation. Unless you need portability, don't bother with external audio interfaces (Firewire or USB) and get yourself a decent PCI/PCIe one. One of the best (but expensive )choices would be RME (either HDSP 9632 for PCI or AIO for PCIe) but something like M-Audio Delta or Echo Audio should be fine.
4. If you want to use Win7 64bit make sure that there are stable and mature drivers otherwise you might get issues with Cubase (or any other DAW).
5. Leave even more money to cure your addiction when you start buying more gear/plugins to make music