The D40 is a great little camera and can be had for under £300, it also has the virtue of being the only DSLR camera you can buy new for that price - so go for it.
But no amount of money is ever really enough for cameras. Set yourself a budget and work within that.
But the question everyone will ask is: why do you need a DSLR? What is there about a DSLR that your current camera can't do?