If you're experiencing slow speed it is most likely due to your real (not virtual) hardware. Remember that your CPU and RAM etc. must be divided between two operating systems while you run the virtual machine.
I would recommend trying virtualbox. It is like VMware, but runs a bit faster and is more lightweight in most cases. You can run the open source edition, which should be in the ubuntu repositories, or you can add the Sun Virtualbox repositories for the proprietary (but still free) edition. The proprietary edition allows USB connections to the virtual machine and allows filesharing between the real and virtual machine.
Here is the download page which lists repositories to add or had a single .deb installer if you prefer. I would personally add the repository, or just install the OSE from Ubuntu's repos.
