... I want something that will take my photography to the 'next' level in terms of image quality.....
I shoot a wide array of things. Typically speaking, I do primarily landscape along with shots of people, ...
Well, the golden rule of tripods is that you can only have 2 of the following 3:
-cheap
-light
-stable
I currently have a Manfrotto 190XB from BH that was $125 but I bought it with a tripod head for a discount. A CLASSIC mistake is to buy a cheap tripod only to have to buy a new one later on (we've all done it at one point) but I really like the one I have for now and it surprisingly holds my 100-400mm, XSi, and flash very sturdily giving me tack sharp photos of birds.
I wouldn't discount the Tamron 17-50 or VC version, they produce excellent results. Whilst a bit noisy, nothing major. And you get the same optical quality a the 17-55. It all depends if you are willing to spend the money. If you are not already apart of this forumthen I would suggest joining it! There are hundreds of lens sample archives from people taking pictures with various lenses.
As for the 50mm, I had it, liked it, sold it. I didn't find the focal length useful enough, and while it was sharp, I would consider my Tamron as sharp. The easiest way to see is just to stick your lens at 50mm for a day and see how you get on.
You've got two zoom lenses. Try a super speed prime lens.
I actually just picked up the Sigma 50mm 1.4 for $449. Nice lens.
A review or two that I came across rated the Tamron 17-50 VC version as very soft wide open (much more than the non VC version) which really negates the purpose of buying a fixed f2.8 lens in the first place.
True, but then some lenses need to be microadjusted or even calibrated. Some are just plain soft wide open. I no mine isn't (Non-VC) so I'm happy
You've got two zoom lenses. Try a super speed prime lens.
I actually just picked up the Sigma 50mm 1.4 for $449. Nice lens.
Any front/back focus issues with the Sigma?
Canon EF-S 18-200mm f3.5-5.6 IS
If you're into convenience of carrying one lens instead of two and don't mind losing the extra 50mm at the telephoto end, this lens will serve you well. Then you can sell the two lenses you have and think about how you could expand. May not necessarily be an "upgrade" to image quality but it's plenty sufficient for non-professionals.
May be you can expand your peripherals? Flash, tripod, remote... etc.
I had a 18-200mm and a 50mm f1.8 and most of my photographic needs, which sound similar to yours, were met. This way you won't be carrying four lenses if you choose to get two $400 lenses like it was mentioned.
Also, Canon EF-S 10-22mm f3.5-4.5 USM would be good for your landscape shots. Wide angle lenses are a great travel companion.