I'm humbled that you asked. When you say 80 to 100 tracks I presume you mean 80 to 100 tracks not 80 - 100 instruments IE you're talking about say 40 - 50 stereo instances of EXS with various tracks for different keyswitched articulations within those instruments? OR are you talking 80 - 100 stereo EXS instruments all playing at once. If it's the latter, hmmm, I think the processors could handle it, but I don't think that hard drive or memory will, especially not with what you're talking about. The way to get around the ram limitations of logic are:
1. EXS Manager - a must have merge your library
2. some kind of RAID system, I have 2X250 gb FW800 drives which works for me and my budget which is low. However you sound like you're getting decent work so I suggest a PCI SATA raid system this means a PCI card in your mac probably with dual bus connecting you to at least 4 X 250 Gb drives (I mean at least as in the 4X drives not as in the 250Gb drives, in fact you could have 160Gb drives that would be fine). Stripe them toether in a type 0 array that means they appear as one drive and spread the information equally over the four drives, so if you lose one, you lose all the data - that's fine since you're using a sample library keep all your original material on those drives if you have modified material that you can't restore from DVD's if a drive dies on you store that stuff on another drive - this set up is your speed set up for loading and streaming your large sample merged files from disk - then in EXS in your virtual memory settings you can set the disk speed to fast and your hard disk recording activity to less, that means less RAM usage, more "open" tracks (unfrozen active tracks). If you have this hard disk set up, I think you should have no problems - coupled with retaining your second (you're going to need it) 7200RPM SATA internal drive for audio tracks and primarily, freeze tracks (store your project files on that drive and the freeze tracks should automatically get put there by logic) you should be able to run what you're talking about. You *may* need to freeze a couple of legato perf instruments now and then, but you should be able to freeze up to 32 stereo tracks on the one drive without running into problems. You should be able to run a video track but you might need to recompress it into a low res file that isn't too processor hungry for example I wouldn't want to run any DV format files through logic (can you do that?) but you get the idea, simple files that are there for timing only, and then you can export your final stuff with the high quality file.
You can certainly mix on this set up, but my point was if you are playing the parts in live which I presume you are, then you're going to want to use a setting of 128 on your buffer settings, which means you will get almost no latency when you play an instrument (so you can hear what you are playing!), however, this is very processor intensive to do - no probs for a 2.7 ghz but once you start running 80 - 100 tracks (should be no problem once again) but then turn on altiverb or something like that, you're going to get stuttering and you're probably going to need to drop your buffer settings down to 256 (still playable) or better, 512 or 1024 (at least 1 second delay from what you play to what you hear) which is great for mixing - you can start loading up your altiverbs etc (It's like finding you have all this horsepower left) but you can't continue to play in tracks.
If I need to play in more parts when I'm at that stage, I just ALT+click bypass all the altiverbs, pretty quick to do, reload with the 128 buffer setting, play and parts and reverse that to get back to mixing.
I definately recommend Logic Pro over Express for what you are talking about I don't even think Logic express will run that many tracks by default.
4 gigs is good but 4.5 is better remember 3.7 is the sample limitation for Logic, the system still needs RAM itself and there you have a recommended minimum of 512mb to avoid swapping, and even then, don't run other apps at the same time. For the system you are getting, I think it wouldn't hurt you to get another 2X 256 simms.
Lastly, Fibre channel apple raid WOW if you can afford that, go for it, but it's pretty pricey, the SATA RAID gives good performance and reliability and is affordable for what you get. I don't think you can buy SATA raid off the apple store - are you buying from the apple store - well I don't know about where you live but here that's about 10% more than what I'd normally have to pay in a shop so shop around and there will be someone to help you out with the SATA RAID set up - honestly take as much time as you can to look into that part of things too because I can't help you out having no experience in actually owning one, I've just researched it enough to know that at this price point it's the best thing to go for for what we want. As far as I know, avoid firewire set ups for RAID and avoid Lacie RAIDED drives, they are cheap alternatives and don't give the same performance, if you want the real deal SATA RAID or apple raid of course. If you want to start out without spending too much on this, you can buy a firewire 800 PCI card (don't use the bus in the mac for this has to be a seperate bus) and buy 2 firewire 800 drives software raid them together and see how your performance is but from what you say you're after I can almost guarantee it won't be fast enough.
Finally GOOD LUCK with that system sounds awesome.
All the Best,
Miklos.