To throw my optinion in, it's pretty much the same as the others here. I have a G4 powerbook that's still alive and used for mainly surfing and spotify. I thought about retiring it, but it works... so why not keep it.
My main work machine is a Macbook like yours only older (2006 2GHz C2D w/ bigger HDD and 3GB RAM). I am only now considering getting a new machine as it's getting a bit slow and isn't going to be Mountain Lion compatible. However I won't be getting rid of that Macbook. I'll put a smallish SSD in it to boot the speed up a little and a clean install of Lion it will be a perfectly fine machine for web, email, light photoshop use for as long as it lasts for.
We do get the best deal in the long run... Macs cost more money, but the components are high quality parts and on average live longer than the PC counterparts.
Keep that old laptop alive.
Yes, you can buy a PC laptop for £250, but you wouldn't want to.