When you need to host several virtual machines (VM) in a PC, the CPU must support virtualization.
Good performance is reached, when there is a logical CPU and a Disk present for each VM.
If you choose a Quad Core CPU, you – as the name tells – get 4 physical CPU’s.
If the CPU supports hyper threading the number of logical CPU’s will be doubled.
The Core i CPU’s is a mess of cores and threads. Some of them supports hyper threading, but not all.
If you choose a i7 for desktop they all have 4 cores and hyper threading (= 8 logical CPU’s), but they are also almost double price of the Core 2 Quad CPU’s Q8400 or Q9500, which does not support hyper threading.
For laptops currently you only find the i7 720QM with 4 cores that supports hyper threading. That one can be found at Dell as the model Studio 1557. This one can be configured to a full HD screen. A cool alternative to a desktop.
Update: Check out considerations for a replacement.
After choosing a CPU you need a chipset. The chipset also needs to support virtualization. The chipsets set yet some other limits like Graphics card included or not. You also needs to decide how fast and how much RAM you want as max.
For the Core 2 Quad CPU’s you could choose chipset Q35, X38, X48, P45, B43, Q43 or Q45
HDMI, Firewire or other plugs wanted?
Do you want Solid State Drives instead of Hard Disk?
The throughput for 15,000 rpm HD’s are in average 175 MB/s whereas for SSD’s it is 220 MB/s. If you choose the HD’s then you get much more space for the money, but do also consider that they need to be defragmented to regain speed. Also consider the fast HD’s are very noicy.
The SSD’s might not store the data as long as the HD’s.
If you buy a PC with HD, you can use a SSD upgrade kit to exchange the HD. That could be interesting if a laptop comes with space for only one drive. Prices are currently around 200 Eur for a 128 GB Drive.
Checkout this vid where boot time is shown for a SSD vs. a 7200 rpm drive.
You might want to add some SD RAM. Some laptops use size SO-DIMM. For desktops it will be size DIMM. They will be different whether you need DDR2 or DDR3 RAM. Yet also you need to know the transfer rate needed. Often you will see it as PC-XX00, where XX is the number you need to know (MB/s).
When you choose a monitor and GPU you also needs to consider screen resolution.