Personally, if I had the money you've got, I'd get a New MacPro and a Fireface 800. If you have Cubase SX already, you could wait for version 4, and upgrade to that, or sell it and get Logic. I'd do some serious research on the interface and computer before spending any money personally. Check out http://www.gearslutz.com for some good discussions about the fireface. Even if you decide on a pc, the fireface 800 is supposed to be one hell of a good unit, if only I had the $$.
I switched to mac this year with a macbook pro and logic pro and am loving it. This system is more like a tool that works the way it should than some cranky child I have to babysit all the time.
Logic is the hardest environment I've used, at least compared to Pro Tools le, and with some training videos from macprovideo.com and some apple pro training books, i'm coming around. Good luck, I hope you get some useful replies.