There are two options really. The first is to use VMware Server, a free program that will let you run Linux Virtual machines with two virtual processors each (therefore on a quad core, you can run two Linux VMs). Its quite easy to set up - just download VMware server, install, download a Linux distro of your choice and install the client as per Stanfords web page. If you like, i could make a quick guide and post it (i'll be reinstalling VMware soon).
The second option is to use Hyper-V, a hypervisor produced by Microsoft. This has an advantage over VMware Server in that it allows for 4-way virtual SMP (allows the Virtual machine to 'see' four cores). Obviously there are some upsides to this, particularly for the i7 chips with '8' cores as it allows you to run just two VMs on one one machine to use 8 cores. However, there are downsides... Hyper-V is only available (well, its included with) Windows Server 2008 and Server 2008 R2 (Windows 7 Server edition basically, currently at the RC1 stage like Windows 7), so you need to change OSes to use Hyper-V. It also has pretty poor Linux support - its mainly meant for virtualising Microsoft OSes, and particularly mouse support in Ubuntu is very poor.
For a quad core, two Linux VMs using VMware is the best option. On my Q6600 clocked at 3Ghz, i was getting around 2500 ppd for one client, and about 2300 ppd for the second, which shared one core with the graphics card client. Upping the clock to 3.3 gave around 2700-2900 ppd per client. I think it is unreasonable to expect to achieve as much ppd from a 4 core processor as an 8 core i7, but the difference should not be that much - a few thousand ppd maybe, depending on your clocks obviously. I tried running Hyper-V and a single GFX card client on my previous, Q6600 based PC. It ran fine, but because the GFX client uses part of one core, it has the knock on effect of slowing the Virtual machine, which is obviously more sensitive to core usage when using four cores.
Another thing to note is that the VMs are a bit of a pain to turn on/off everytime you shut down, but because your a 24/7 type, this shouldnt be a problem. There are ways to automate the process, but its still a bit of a pain. RAM usage is also a bit of a problem. The guys on the official F@H forums recommend at least 1gb of memory for each virtual machine running an SMP client to reduce the usage of the SWAP (page) file and speed up the client. In reality the client will run on a much lower level of RAM, but i would not advise running VMs on a machine with less than 4Gb, just because it eats into RAM available for the system and other running applications. Its a bit of a pain if you plan on actually using the PC, rather than just letting it sit there folding!
Phew, long post, but i hope it helps. I believe there is a link to SiriusB's guide that rich includes with all the news updates, but obviously if you have any questions, i'll try and answer them. Linux VMs are definitely the best way to use a processor to gain maximum ppd, and really i have had minimum problems running Linux clients.
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I actually seem to be producing less with the new rig than the old - must fold harder!
