OK, theory first:
Digital effects are digital effects, which means that you can use the exact same algorithms in plug-ins as in outboard effects, and therefore they can sound exactly the same. Analog effects are always hard to duplicate exactly, and some more than others. The hardest ones to duplicate seems to be tubes and speakers.
Now the reality:
Different outboard effects sound different. So does different plugins. You'll probably be able to find plug-ins that sound better than some outboards, and you won't have to do an digital to analog to digital to analog to digital conversion when using them. On the other hand, the best algorithms are proprietary, so you wont get that perfect smoooth reverb out of just any plug-in. To get a lexicon reverb you need lexicon, and I think they only make a plug-in for Mac ProTools...
So basically plug-ins doesn't, in themselves, sound any better or worse than outboard effects. If outboard effects sound better, it's because the makers of these have been around for longer and so know what they are doing.
If you are using a HD-recording system, go with the plug-ins. But just as with outboard effects, listen to them first. Besides, using outboard effects with HD recording means that you have to have a lot of inputs and outputs...