Some tips...
PD5 is pretty damn fast but you have to treat each VM like it's own hardware install. The less resource sharing you can get away with the better your computer will perform.
1. Get more ram. I assign 3GB to my Windows VM. Plenty for OSX left (5GB), plenty for Windows. Unless you are hitting 2+ Windows you're still going to deal with thrash/swap during normal usage. As you've got a 3GB total Mac, you just don't have enough to satisfy both OSX and Windows requirements. Ideally, get 4GB and give 2 to each OS.
2. Keep your VMs on a separate physical drive. If you're reading/writing something in OSX and your drive is capable of a 50MB/sec transfer rate, great you're traveling at 50MB/sec. Now if you're going to start doing something in Windows also disk intensive, you're now (more or less) halving your possible disk speed.
3. Give your windows 1 core. OSX is pretty smart and will see that one core (the windows one) is working more than the other and do most of it's work on the less tasked one balancing out the load. If you give both cores to windows, you're now at the mercy of 2 different schedulers effectively trying to fight each other.
My system.. 4 CPUs -2 go to Win VM, 8GB ram - 3GB for Win VM. I also run Windows on it's own 320GB drive (sata, but USB or Firewire work fine). The GUI can still feel occasionally slow, but the video card is really the only device that has to be shared between the two systems. Everything else is as compartmentalized as possible.
I do a lot of media work and my rendering/compiling times are pretty close to what I was getting under Bootcamp.