If its only 2 computers, you just need a crossover cable. to link them together. give the computer that is connecting to the net an ip address of say.. 192.168.0.1, and tell the second computer to use that as the default gasteway..
TEchnically yes your download speed will be reduced, but with it being adsl... not much.. normal internet surfing and stuff you wont notice a difference. if its a 512k connection and both of you are downloading a huge file at the same time, each will take 50% so instead of the file comming in at 50k a sec youll only get 20k a sec