My first question would be do you shoot in raw and do you have Lightroom ?
If you don't, try this first, including buying a book on Lightroom... Next up, check at what focal lengths you shoot at most, this is easy in Lightroom... I'd first consider a better APS-C lens, i had the 15-85mm, but the 17-55mm might work for you, my photos kinda looked a bit better, next up I'd consider a low f stop prime, I got the 50mm f1.4 new and a secondhand 135mm f2.0 L prime, my photos started to buzz with these. I added the 10-22mm which was fun to use and gave me some great images... I also got into using ND grads, polarising filters and black glass.
My story drifts from here, as I took the plunge with a 6D when it was launched, bought a 16-35mm soon after, then a 24-70 f2.8 II soon after that.... I sold my 450D which ended up costing me £50 per year of use, bargain ! Sold my 15-85mm which had cost me about £60 per year of use.... Still need to get shut of the 10-22mm and although I'm pondering swapping my 70-300 non L for something better, I'm still getting good images from it, just annoyed with the buzzy AF it has and slight loss of image quality beyond 200mm (pixel creeping necessary to see)
Moral for me is don't jump in at first, learn what you need through Lightroom, play with light using filters and once you get to know your photos, use that knowledge to improve your kit sensibly.