Personal;ly I would buy them all from one supplier...click here is the one that I would choose and it saves all the messing about trying to save a pound or so here and there. All suppliers are about the same price.
Between them I can normally find everything I need at reasonable prices, and have never had any problems with any of them - however I've never needed to return any items, which is normally the real test of an online retailer.
Endorsing GANDALF <|:-)> and his recommended supplier..... This firm is very helpful if you have doubts about compatability of one or another component. It will only cost a short phone call, i.e. if you can't get the necessary information here!
All the ones listed I like, except Ebuyer who I will never use again, like Gandalf Novatech is my favourite. Here is a useful site to get an idea of prices click here but it doesn't check all the suppliers. I would either use one supplier for all your system or two at the most as any money you save using more sites are soon wiped out by postage costs. I built this PC a few weeks ago and used Novatech and Scan with Crucial for the memory click here . I only used Scan cos Novatech didn't have everything I wanted.
Have to agree with the suggestions of Novatech - good prices, quick delivery and excellent after service should you ever need it. I've had a few faulty bits from them but always collected next day and replacements delivered.