It will have absolutely no affect on your download/upload speed on DSL.
Gigabit ethernet is usually used for local networks. Schools and businesses sometimes have these high-speed interfaces, and it makes sense for them to have it since they need to transfer files (sometimes large ones) as fast as possible.
With your setup, the bottleneck will be your DSL upload/download speed cap. It is WAAAAAAAAAAAAAAAY below the speed of Gigabit ethernet, and thus you don't need a gigabit Ethernet card.