Ok, Can Anyone Enlighten Me? Options

[JackC]

I'm about to buy a new system for use as a pro mobile setup. I've got a PT Mix Plus System in my Studio (which I'm probably going to let die because I can't justify giving Digidesign another 10-15K just to stay 'up-to-date'). So... I'm looking at Logic 5 OSX (been using v.4 for a while - extremely impressed with all the features and instruments). I need a Pro front end (I've been using Apogee's in my Studio) and I will probably go with the Prism Dream ADA as it sounds amazing and is completely upgradeable as far as sample and bit rates go.
But which audio card to buy...?
Unfortunately I've been spoiled with ProTools as far as latency goes (by this I mean happily monitoring an input with Plugins on it while recording, while also running up to 40 other tracks with Plugins on them) and I've been led to believe that I can now get this type of performance from a Native system on a super fast Mac under OSX. True?
Is it, or will it be possible for example to record and monitor an Electric Guitar track with an 'Amplitude' type plugin on the channel with acceptable latency while running lots of other tracks in my session?
I've been watching the 'Hammerfall' website with interest, apparently their cards can take on some of the DSP involved in keeping the latency time down. But will this become obsolete with the claims that OSX with it's Core Audio will accomplish near to zero latency with no additional DSP hardware?
I really like the look of the new UAD card with it's ADAT I/O. I was planning to buy a UAD card anyway as I've demo-ed one, and the Plugins to my ears seem better than the Bomb Factory equivalents.
So basically what I'm asking... (sorry for the long drawn out, bracket infested post!) ... can someone give me suggestions for an audio card and... whats the truth about all the latency free recording claims under OSX? Can I get performance equal to my Mix Plus on a new Native system?
Thankyou for bearing with me!