Most places like Dabs, Novatech or Simply (etc.) do upgrade bundles which have everything you need and are a bit cheaper than buying the bits on their own. More importantly, if it doesn't work then it is their problem, not yours. Best bet is to ask to talk to a technical sales support guy/gal (if they don't have one, hang up!) and tell them what your current system is and what you would like. They should be able to help.
One of the more stupid computer things I ever did was attempting to save about twenty quid buying different bits from different people. Put it all together and it powered up but failed on the POST, so I spent about two days on hold to different tech support lines who all denied that it was their kit failing.... <img src="frown.gif" border="0">