I am not sure of the specs of your Edirol sound card, but broadly speaking, the Mac does not benefit from external sound cards or even internal ones, other than having additional audio inputs/outputs.
It's actually worth understanding exactly how and why the soundcard became so important on Windows machines. It all goes back to the days when PCs barely had enough horsepower to draw lines on the screen, much less playback multi-track samples or add effects like reverb. Even when CPUs were powerful enough to do this, Microsoft's poor APIs meant that the PC always struggled with latency and stability issues. If the system's attention was diverted to, say, an anti-virus scan or even some unexpected call to the HDD, everything stalled and your mix/jam or whatever stuttered.
This lead to a huge market in both soundcards (everything from Turtlebeach to Soundlaster) and the growth of alternative APIs, such as ASIO, to basically remove the whole audio loop from whatever else the OS was doing.
Ironically enough, MS actually wiped all of this away with Vista. There is now virtually no advantage to having a $300 sound card in Vista over a generic, integrated VIA chip on the motherboard, in terms of internal sound quality. Again, the only real advantage is the number and quality of audio inputs/outputs. This is probably the one and only great feature of Vista (although they messed it up in the original version, it was fixed in SP1).
Macs use Core Audio, which puts all the work on the CPU - in theory, a Mac Mini is more than capable of entirely creating, mixing and mastering a track of commercial quality for either music, or even film.
I am exceptionally biased, but I would say Logic or Ableton Live are better suited to the Mac than Cubase, which IMO suffers from poor workflow. I started on Cubase on the Atari, went as far as Cubase VST 4 on the PC, and then switched to Mac and went straight to Logic. Logic Studio is also far better value for money, considering the massive sample library you get with it (more than 40GBs) and the instruments that come for free.
Having said that, your Mini will handle Cubase with no trouble at all.
