Replying to Lens help.

ok so i recently got my first dslr(canon rebel xsi) and i'm looking to upgrade lenses. Obviously i don't want to use the kit lens amymore, so I'd like to get a new one as soon as possible. However, like many of us, money is an issue. I was looking at the Canon 50mm f/1.8 and from what i heard it's a great lens for the cheap price. There are also a few wide angles/fisheyes i was looking at for a more expensive price. What my question was is should i buy the 50mm now and use it to shoot with until i get the fisheye or wideangle?? or should i just go for getting the fisheye or wideangle right away? and if so...any reccomendations?

I know it's kind of all up to me and an opinion question, but i'd just thought i'd ask what you guys thought.

Thanks
+k to all those who help!