Your camera is decent, and great photographers can take some amazing photos on cameras, polaroids and "crappy cameras" in general. I have a similar set up (xsi), the kit lens, as well as a super telephoto that I love. I love the reach of my super telephoto, so I tend to use that as my walk around lens (which gets lots of laughs from people). However, the 18-55 lens is very good and can produce some great pictures.
Personally, I've already seen some improvement in my photos since I bought my camera about a year and a half ago. It's not having the "better" lens since I bought it at the same time as my camera. Instead, it's been practice, reading the advice in all the threads, and posting pictures for critique and actually listening to what they have to say.
As far as your camera being upgradeable, if you're going to invest in your equipment, I'd invest in a tripod since your interested in night/storm photography. There are lots of opinions on tripods and several threads about what's good and what isn't (lots of differing opinions, so you need to go with what works for you). Also, you could eventually get better lenses, but it's not necessary.
Fast lenses are great for dark and night time, but if you are shooting lightning, you'll be using long exposure times anyways, so the lens you have should be fine. But, the long exposures are why you'll need a tripod (or a stable surface to set the camera down - the poor man's tripos

).
If you want to upgrade lenses, think about what you want to shoot. I wouldn't get a new lens until you find something you absolutely can't shoot with your current lens. Once you figure out what it is, that will let you know what type of lens you will need to buy.
I think your set up is fine, so get out and shoot stuff! Looking forward to seeing some of your work.