Well, kinda/not-really/yes.

I assume you know what DOF is and how it works? If not search here for that or on google. There are some great interactive web applets and tutorials with examples and etc. on the topic. Anyway, yes the 1.8 will allow you to shoot a little faster in low light but more importantly it'll give you (the ability to achieve) dramatic DOF blur. This same principle additionally allows you to isolate a subject (good for all kinds of stuff but people always mention portraiture 1st) by blurring the BG and FG elements. Something like this maybe:
Mav is also saying that the glass and design are optically superior to the zooms you now posses which I believe is true. Actually most primes are "optically superior" to most zooms but... Anyway that's the short of it I guess.
On the other hand with a flash unit and a $30 AC close-up lens you can do this
http://thephotoforum.com/forum/showthread.php?t=126794 with the lenses you have now. It all depends what
you wanna do.