The differences between looking after a hamster and a pair of rats are significantly important so it's a good job you asked! You've already obviously done a fair bit of research as I'm glad you've opted for a pair of rats over a singe one - so many people seem to getting single rats nowadays and it's just so sad that they aren't allowed to be sociable!
Housing - your rats will need a significantly larger cage than a hamster. The recommended welfare standard is 2 cubic feet per rat, so that's going to be 4 cubic feet for your pair which will be a pretty large cage to you if you're used to hamster cages! For a pair of rats I would recommend the Savic Freddy 2 Cage as it's an ideal size, very good quality and easy to clean. Most pet stores sell this cage, retailing around £70, but you can easily get it online for much cheaper - it's easily recognized by it's yellow base and navy blue bars. When buying your new rat cage you must take great care to ensure it's the right size - Ebay is not a great place to look unless you know the exact name of an ideal cage - there are countless people selling cages that are TOO SMALL for hamsters, let alone rats, as rat cages so you must try to get your cage, or at least the name of a cage from a responsible rat breeder or pet store - putting your rat in a hamster cage will cause growth and behavioral problems and even health problems related to ammonia levels.
Food - like hamsters, rats are best fed on a cereal diet specially formulated for rats - you must take care to ensure it's a rat feed as many foods designed for hamsters, mice e.t.c don't contain enough nutrients to keep a rat healthy and often contain sunflower seeds which rats are allergic to and shouldn't be fed. I would opt for the cereal diet over Lab Blocks, as they provide more texture and variety to the diet, however you must feed no more than the recommended amount per rat as suggested on the packaging or they may pick and choose which cereals they eat causing a lack of certain nutrients. You can supplement your rats diet with biscuits or fruit and vegetables every other day - but feed them in moderation or they will make your rats ill. Rats, like hamsters also need something to chew to keep their teeth down so constantly providing a bone-meal dog biscuit or untreated wood block is ideal.
Care - rats need a full clean out every week, with you taking out damp patches, poop and mopping up urine everyday. You should use shredded cardboard (dust controlled specially for small animals) or a bedding like Carefresh for your rats as some types of wood shavings are extremely toxic to rats, but nearly all of them cause respiratory
problems in rats so I wouldn't recommend them. Rats tend to be more friendly and sociable than hamsters, and aren't nocturnal adapting to your life style so they'll probably need more out of cage time play and cuddles then your hamsters did - at least an hour a day, but more if possible - an ideal place to play is on the sofa if you put down some mats in case of accidents - your rats will love to play all sorts of games and will probably tell you when they want to go back by acting restless or showing signs of needing the loo!
One of the advantages of rats over hamsters, is that they will smell less!
I would recommend you get a few books on rat keeping, or go and visit other rat owners or breeders before you get your rats as that way you'll know exactly how to care for them when they arrive. Rat books will also explain to you the best way to settle your rats into their new homes and get them used to you and handling.
Hope this helps!