This seems to be a mis price as its showing £4.53 discount but the price was originally over £38.00.
Its 800MHz and at least you know it will work with your XPS 420 as many have commented on other brands not showing correctly.
Dell are charging £20 extra for 1GB of 667MHz memory when buying a system.
2GB 667MHz also available for the same price if matching up with your system current spec
Supercharge your Dell XPS 420 Desktop with a Dell Memory upgrade engineered, tested and customized for your system. Dell Memory is another great product from Dell high quality at an excellent price
Don't forget quidco for further discount!!