I presume you're in the UK? I'd advise you read the customer reviews at Newegg, although it's US based. The reviews typically are numerous and can be helpful, so long as you separate the wheat from the chaff.
That's what I do when considering parts I've not used previously. Make very certain your CPU and mobo are a good match, and don't go too cheap on the power supply.
Here's the page for the Q6600:
http://www.newegg.com/Product/Produc...82E16819115017
And the E6850:
http://www.newegg.com/Product/Produc...82E16819115028
There are lots of guides on the net to help you build a computer. I think this one is very clear, and there are good photos of each step:
http://pages.suddenlink.net/sredwudz/index.htm
Good luck.
[EDIT] I see both CPUs are 65nm architecture, that's good, they should run (relatively) cool. Both are EM64T capable, meaning they'll run a 64 bit OS. Make certain the motherboard is EM64T. Both CPUs are socket 775, you'll have plenty of mobos to choose from. I'd suggest getting a mobo with PCIe slot for graphics. Pay attention to cooling/fans. If your mobo is ATX form-factor, get an ATX case, that should be self-evident, but it's easy to miss details. You'll figure it out, take your time and methodically match up parts, starting with CPU and mobo.