Peter I never looked into the NX card as I just had a Quadro FX 560 which lets me preview when it comes in and outputs to a HDTV or whatever anyhow. Go an give EDIUS a go by itself then make your decision about the NX card.
Other more powerfull graphics cards give you very good GPU so allot more effects are realtime.
I always firewire in and one can always capture with the DV camera from an analog source if needed, then in.
AE can handle the Canopus HQ codec so no problems there from what I hear.
Make sure you download ProDad's plugins, they really work great. Their Vitascene is very good with a powerfull GPU card and Heroglyph is reall cool for easy text effects and more..
A 55 min HDV timeline took about 2.5 hours to render out to a 8.5mbps CBR progressive DVD spec MPEG2. Qualtiy is wonderfull. The m2t file took 3 hours to make then use the TS writer to send back to the camera when you are ready.
You will get a laugh editing DV and HDV (CanopusHQ) together, it's so easy then just render it out to whatever with Procoder Express even QT formats. Upconverting DV is a snap but it never looks as good as the real HDV, but you can sneak in the odd bit here and there.
It does take some time mastering pips and graphics around the screen though and at the start you will go what the *&^%#$... 
It does not do AC3 but thats not a problem for us PAL mob as all PAL dvd players can play the tiny MPEG sound files. NTSC players cannot for some reason.
To make your DVD's DVDLAB PRO is cool and very powerfull. I even burnt our master this time with it's built in burning software, did a verification with it and then sent it to the replicators. We tested it on a old G4 powerbook laptop, old PC laptop, desktop comptures and one very fussy SONY DVD player to a normal 4.3 TV and even out to our Epson TW 700 projector from it . Played and looked great.
Good luck