Jaz,
Since I still have XP on my drives and all of my programs have been optimized to work with XP, I configured my machine with a socket 775 mother and processor. They work well with XP and are cheaper than the i7 and i5 processors/mothers. The link for that is
http://www.avadirect.com/product_details_configurator.asp?PRID=2163.
For the socket 1366 i7 processors, you would go to
http://pc-barebones.avadirect.com/PC_Barebones/Socket_1366_Core_i7_PC_Barebones/.
For the socket 1156 i7 and i5 processors, you would go to
http://pc-barebones.avadirect.com/PC_Barebones/Socket_1156_Core_i7_and_Core_i5_PC_Barebones/.
Both are a bit more expensive than the old socket 775. I chose a 65W processor to reduce the demand on cooling, but still got a good speed (3GHz). Both types of socket can be configured to work with Vista and Windows 7, and some of the processors can be configured with XP Pro. All these links would give you a pile of processors to choose from. Just make sure that the cpu speed is supported by the mother that you choose, and make sure that the ram will work well with both. If you make a completely incompatible selection, AVA will contact you before they assemble the system so you can get the details ironed out. They're good people to work with, and they do a neat job of assembling. Adding the cost of what I already had (pair of 500BG drives, modem, sound card, cd/dvd rw) to the chassis that AVA put together for me gave me a system that performs as well as the Area 51 that I had, is infinitely quieter, and cost about a third of what I paid for the Alien.