Dell are hard to upgrade cause of the case and motherboards they use although some models are easy to work on.

Shings what do you want the pc to do? ie.. games, internet, video encoding etc...

http://www.pcworld.co.uk/martprd/product/seo/359879 Not a bad spec but please note that the mobo will not be a big branded item, probably something like trigem or budget msi board.

You have to remember that pc`s are built to a price point so although you see a similar spec pc costing £200 more than another brand it will probably have better quality parts in it.