My experience was that 50 USD notes is the best. 20 USD notes gives a lower rate and 100 USD notes creates some suspiciousness regarding fake money.
Fondest memory: There are several ATM's in Lilongwe. Most of them take VISA. Mastercard is however not that easy. I didn't find it, but according to other websites it is a ATM for Mastercards at Lilongwe Airport. ATM has a daily limit of 20,000 MWK, which is 140 USD at official rate. Note that there is a black market of money changing in Malawi. You will get the lousy 15% lower rate at ATM's compared to the streets.
Fondest memory: Lilongwe is the commercial center and capital of Malawi with a population of 900,000 people. It lies in the central region, on the Lilongwe river. The town is marked by signs of colonial settlers. There are two distinct parts of the city. The Old Town, which is the original part of the city and features busy African markets, many Asian shops, hotels and restaurants. And the New Town, which is less than 20 years old and hosts the Government Buildings, Diplomatic Missions, International Aid Agencies, etc. There are some new shopping areas, hypermarkets, hotels and restaurants.
Fondest memory: Lilongwe is diveded in an old town and a new town. It's a city with both gravel roads and bitumen roads. Houses in the the city is mixed from dilapidated houses with flat corrugated roof and modern concrete buildings. The pavements are in use for street sellers.
Fondest memory: Electricity in Malawi is 230 Volts, alternating at 50 cycles per second. See photo of the outlet shape of the plug.
Favorite thing: Lilongwe is not really a city designed for tourists. Its more an arrival or kick-off point for the Central Region. Don´t expect too much!