I run three G5's : a Quad 2.5 as master, and DP's 2.5 and 2.0 as slaves,along with a P4 PC, running GS 2.5. All the slaves are fitted with Hammerfalls, which are connected to the Quad's 3 MOTU 2408 Mk 3's and a 24 i/o. I have V-Stack running on the DP 2.0,and although it's a bit cranky under Tiger (crashes without fail whenever i quit it) it performs very well, generally. It is simplicity itself to use. The Quad has 8 GB RAM, whilst the other two G5's have 4. The PC, 1.5.
I've just bought Kore for the DP 2.5, and that's great,despite there being a few bugs, which I guess are to be expected. I've not bothered with MIDI hardware on the DP 2.5 (I do not want another bank of AMT's!),so I've been setting up MIDI over LAN, using the Mac's own AMS. It works very well and, in principal, can yield as many ports as I'll ever need, for no extra outlay. i'm very pleased with this: it seems rock solid, after my initial familiarisation with it.
V-Stack is fantastic value for money: never experienced any dropouts, and the interface is excellent; but I would seriously look at Kore. There have been a few dissatisfied users on the NI forum, but I notice that they're overwhelmingly PC users. [:D]