A passport is the only requisite for citizens of the US and EU member nations for stays up to 90 days. This can be extended once for a further 90 days by the Immigration Department.
If you have any questions, get in touch with your local Argentinean embassy. The UK office can be found at the following address:
Argentinean Embassy in the UK
27 Three Kings Yard
London W1Y 1FL
Tel: 0171 318 340