I would suggest that some of the decision should depend what you use the PC for. You are asking about PCI-E - so I assume your old PC is AGP. If you do anything more than basic graphics you will need to allow for buying a graphics card as well. Most motherboards have some basic graphics built in, but not all, so check before you buy. If you play games you will want to spend more on the graphics card, and a bit less on everything else. If you just use office type applications then it won't matter too much what you get. If you do a lot of video or photo editing then you will benefit from more cores in the processor, as long as your applications can make use of them. Things like Photoshop, The GIMP, or CAD software will use all the cores, but most applications won't necessarily, so if you don't do this kind of stuff you might be better off going for a faster clock speed with fewer cores. AMD generally give you more power per £.
My retailer of choice at the moment is <a href="http://www/cclonline.co.uk">CCL</a>. They do some motherboard, CPU and RAM <a href="http://www.cclonline.com/product-categories.asp?category_id=491">bundles</a> which might suit.
