I agree that you need a great mic, and a great preamp can help too(you need to decide how much money you can afford for those). The Profire 2626 has a reputation for sounding good and I see no reason to steer you away from it(except that it does not work with LE). Now, having said that, you may be just fine continuing with Cubase. I use and love Pro Tools, but its not the only choice out there and no DAW is perfect. You can get PT M-powered to use with the 2626 and that would allow you to learn Pro Tools, while still being able to work with Cubase. I think that sometimes, its harder to learn any DAW software after using another one for a while(I can't do anything on Cubase and feel like an idiot staring at the screen)
As to how well your computer would work with LE or M-powered, it should be okay, but it won't be a "power house". My old Athlon 6000+ would score 104 on the Davec test, while my i7 quad scores over 300
. But I was able to run 30-40 tracks at the 1024 buffer.