some links, in case you didn't know:
Sounds Online (EW home)
VIENNA SYMPHONIC LIBRARY (VSL, obviously)
Westgate Studios (westage, obviously again)
Sample Modelled Instruments - Home page (home of THE TRUMPET, the most amazing synthesized trumpet ever!)
Kirk Hunter Studios (kirk hunter, another very good symphonic libraries creator)
Wallander Instruments - Site is under maintenance (brass to die for, woods, so and so imho. Walllander Instruments, all synthesied)
Pianoteq – True modelling of pianos and other keyboard or string instruments (true modelling and a piano that rocks at 15 MB!!!!! (not GB))
ProjectSAM Cinematic Sampling - Home (Project SAM, with the newly released Symphobia, a pleasent different style library, I think. Also some blood amazing percussion)
Garritan Home Page (we all know who Gary is and plus his Steinway is simply stunning!)
PLAY is the new player by EW.
Kirk, Project SAM, Westgate and maybe more (not sure about those 3 either, do check), do NOT have a player of their own, which means that you also need a sampler to work with their fabulous stuff. EW, VSL, WIVI, Pianoteq, all have their own players, so no sampler is needed!
Legato: Think of it this way: You get a violin. He plays on the E string and then slides the finger to the next note. This is called portamento in string playing. Or glissando if you want (although glissando is a different idea alltogether). This CANNOT be done successfully with samples at this time. I mean you get 2 different recordings. The 1st and the 2nd note. The problem is to connect them. How are you going to do the portamento? Some companies (VSL, primarily) has RECORDED various speeds in ALL intervals for ALL instruments. So the full package (which I think is $12,000 but not sure, Nathan, haven't checked for more than a year), is (or was, since I've not checked for quite a while) around 750 GB of hard disk space...
EW and their new player PLAY, have some kind of emulation, which I've yet to hear and yet to try, so I cannot say anything. Kirk Hunter has some great stuff for Kontakt (some scripts), which emulate pretty well the legato issue. WIVI and the trumpet (samplemodelling), are synthesized and NOT based on samples, so they can create realistic legato movements.
4 GB of RAM might be too little in a little while, I'm afraid. The monitors you have are not bad, but at some point, no matter the relation money/value, you might need better stuff. I mean EW Platinum is 24-bit. In order to hear this you need a good soundcard to support 24-bit and some rather good monitors. You won't hear it with some mp3 headhpones (for example, the Mackie are good, I repeat that).
Since you have a top budget of $4000, you might consider other ideas as well. If you do have access to various players, you might consider some good mics and a good converter (AD and DA). If you have access to a university studio, never mind really, but at some point you might realise that no matter the samples or your DAW, live playing IS live playing and if you are blessed with people who can play well, then use them with all your heart!