Bigger is better?

[Xarea]

I was originally getting the 20-inch: 2.4GHz iMac, but now i've decided to take the plunge and go up to 24-inch: 2.8GHz.

Apart from the size and processor difference, what else am I getting for my £300 extra?

[Recursion]

Not much else, I think you get a bigger hard drive and thats about it.

[lolwut]

A higher quality in your display. There's speculation that the 24" has a higher quality display: http://blogs.chron.com/techblog/archives/2007/09/an_imac_display_dilemma_20inch_value_or_24inc.html
Watch the embedded YT video, you see why most people go for the 24" iMac.