In my experience, MAC'S just seem so much more stable and straighforward to use than PC's. Windows IMHO seems to be a weaker copy of the MAC OS (sorry If I've offended anyone here!). However, fact remains that PC's dominate the computer market for business and home use. They are alot cheaper than MAC's but fact remains that when you buy a MAC you are buying everything from the one place and not a whole heap of parts put together from all different places!!!! Yes it might seem I'm anti PC- I'm not Pro PC or MAC but my many times pulling my hair out using a PC for midi/audio work has given me a bit of a sour taste in my mouth for using them in any serious recording stuio application. My advice- buy a MAC for the Studio and a PC for home.
Yes Digital Performer is also very powerful (and also underratted unfortunately compared to Logic and Cubase). I will check out DP but I know Logic quite well. Cubase has never impressed me although I know many people who use it- it's overly colourful IMHO and too gimmicky in it's approach to doing things. However, Logic, Cubase and Digital Performer are all very powerful programs and you can't go wrong with any of them. Opcode's Studiovision is also great but it's future looks dim at present! If you want something really poweful midiwise but more limited in audio (mabey something to hook to something like Protools) check out Cakewalks Metro 5.0- VERY POWERFUL MIDI FEATURES FOR A CHEAP PRICE!
As for audio cards- if not requiring 24bit audio, Korgs 1212 card offers alot for the money with it's ADAT, analogue and SP/DIF I/O and ADAT sync capabilities. Also check out MOTU's 2408 mark 11 for more upmarket work. Plenty of other options also exist.