As mentioned previously, I'd look at getting a macbook pro.
Apple do some good deals on refurbished units. At the mo, there is a 15" pro with a slightly better spec then the Imac you are looking at for £1300.
http://store.apple.com/uk/product/FC...mco=MjE0NjE5MA
Its worth it for the extra ram and hard disk space. Plus the ability to use the expansion bus for a 2nd dedicated firewire card...oh and portability as well.If you need the large screen and external keyboard. You can hook these up afterwards.
My advice when buying Apple products is to take out the Applecare scheme when you purchase the mac. It's a lot cheaper then doing it afterwards, and believe me its a lot cheaper then forking out if anything goes wrong after 12 months. So far my mac has cost Apple more to fix, then it did to buy a new one. Luckily, I took the plan...so they are paying for it !