You don't gain much by building your own PC these days apart from added complexity when troubleshooting.
But if you want to do it as a learning experience... Well, who am I to argue apart from wish you good luck.
First thing that comes to mind is ... Don't forget a decent anti static kit.
Second thing is £89 looks too cheap for a decent graphics card.
Third thing is you don't list manufacturer part numbers, which makes your part descriptions somewhat vague.
Fourth thing is you probably need to check the license terms on that copy of windows 7 OEM you are thinking about. You may find Retail FPP is what you really ought to be budgeting for.
Fifth thing is you might be better off with one 4gb memory chip... Fairly pointless going multi chip for such a small quantity of ram. Also means at a later date, you have a better upgrade path... Ie add another 4gb chip give you 8gb