If you plan on staying with Canon, I recommend that you invest in L glass starting with the 17-40 f/4 L. It's about $750 bucks, but it's a great lens. I recently picked it up, and it's on my camera more than any other lens that I own. Also, get a 50mm 1.8, or 1.4. The 1.4 is the better model and chimes in around $350 or so. I own the 1.8 and want to upgrade to the 1.4. The 1.8 can be had for around $100. And one more lens to add to your camera bag that you will absolutely love, is the 70-200 f2.8 IS. It is the most expensive of the three around $1500 or $1600. But, you may be able to find one for less since Canon just recently released a MKII version that now sells for ~$2400.
I know it all sounds expensive, but you have to remember that bodies depreciate and glass pretty much holds its value. As long as you have the good glass, you're going to have photos that have much better color/contrast than cheap lenses. Also, when you start moving up to higher megapixel bodies, you're also going to start seeing more noise in photos, which will only be more pronounced with lesser glass.