I don't whether it's my Powerbook or Digital Performer 4.1, but a lot of the music I've recorded on both has a rather cold, 'digital' sound. Does anyone know the best way to improve this?

My biggest problem with DP lies with the plugins. I can't get the reverbs to sound any good at all. And I just can't get anything close to a lush, 'full' vocal sound either.

Can anyone help?

You don't mention how you are getting your tracks into Performer, but my bet is that it is not through a quality preamp. You can record the same track through the same microphone into the same system, but changing the preamp that mic goes through will change the character of the recorded track like night and day.
If you want warm, lush tracks, your talking about using a high quality tube preamp. For the most part, in the digital workstation world, when you want warm, you want tubes. And if you want rich, lush sound, you're probably also talking about investing several hundred dollars (or even a few thousand, for the pros) on a quality preamp, or more likely a 'channel strip', which would include some kind of compression or limiting or vocal processing or even an adjustable 'tube warmth' setting. The preamp is not the place to go cheap, a great preamp can make cheap microphones sound great.

As far as reverbs go, Altiverb has done wonders for me.
The Apple AU reverb that comes with DP4 is noisy and harsh.
Altiverb is about 6 or 7 hundred dollars but it has what is called 'impulse responses' that simulate verious acoustical environments.
With my humble G4 dual 533mhz processor I just use an SM 57 and the vocals are perfectly acceptable.
A nice preamp is next on my wish list and will do wonders to improve my sound.
From what I hear Presonis has an affordable preamp that should do the trick.