Can't speak authoritatively about your dell, but a new iMac, even the base model i5 27" will be a decent speed bump. Going for the faster model or the i7 more so, just based on a more modern faster processor.
One note on the RAM. On the 27", the RAM IS user upgradeable.http://support.apple.com/kb/HT5540?viewlocale=en_US&locale=en_US
What's more, the iMac comes with the DIMMS in 2 slots, and 2 more slots empty, so if you buy the base RAM config at 8GB, you can buy another 8GB for about $100 (UK pricing should be somewhat similar, which is half what US Apple store charges to go to 16GB, although again they only fill 2 of the 4 slots. So you might think about bumping the RAM yourself and using the 80 pounds or so you save towards other upgrades you can't do your self.