In General, it will depend on the number of VMs you'll gonna be running at the same time.
If you're gonna be running 2 or 3 VMs simultaneously and you're gonna install web server
applications on those VMs or if you're gonna be using it to run demanding network applications
or programs for testing and benchmarking and you're gonna be hooking it up all to a network,
a fast quad core procie will handle those tasks.
Since you're eying on that AMD Phenom II X4 945 Deneb @ 3.0GHz. Its is enough
for your VM'ing needs not only it is much cheaper than Intel's i5 procie that you indicated
above as an option but you can overclock it way more than you expect.
And Overclocking is the middle name of this AMD procie coz you can overclock it up to 3.8ghz,
thus giving you noticeable performance increase.
Just make sure you have a good well ventilated case and a good CPU cooler.
If you're not into overclocking, not a big deal, as i said, its enough for your needs.
As for the VMs to run properly based on the requirement of the operating systems
you're gonna install and to make sure your main OS will run smoothly at the same time
adding more RAM to your system is highly advisable. You mentioned Ubuntu 64 bit
and Windows 7 64 bit, in order to run Windows 7 smoothly you'll need 2 gig of ram
and for the VM (Ubuntu) you'll need 512mb of ram assigned to it.
My sugestion, go for atleast 3gig of ram but IMO, buying a 4 gig of ram is
much much better decision because your system can be able to run 2 or 3 VMs simultaneously
without using the reserved advisable 2 gig of ram that your main OS is utilizing.
Hope this helps.
-- I hope you wouldn't mind me asking, you're getting certification on Linux right?
Is it for Ubuntu 64 bit server edition? Why Ubuntu?