You should remember you can always sell / trade old games so in reality they'll never cost you the full £40. Would I say Kameo, Condemned, COD2 and even Kong were worth £10? Hell yes.
But... if it's longevity you want it has to be PGR3. I bought it on launch day and it's the only title I still play regularly. I'm not sure about DOA4; it's fun for small bursts but imo more of a party game than something you'd want to play all the time. Plus you'd have to start visiting a physiotherapist regularly for your knackered hands if you did.
You should also get Oblivion on pre-order, that'll keep you busy if morrowind (the last one) was anything to go by.