Buying good memory cards is becoming more important than ever before. Almost all pro level cameras use Compact Flash cards, but a few are starting to accept Secure Digital (SD) cards. They are not as durable, but they are cheaper and easier to find in a pinch. Cards get formatted so many times that they're pretty much bound to fail at one point or another.
Since the RAW files I shoot are much larger than common jpg files, I can only fit about 500 photos on a 16GB card (a little more when using the older MKII). I've heard that when using the 36mp Nikon D800 you can't even fit 500 files on a 32GB card! My full weddings range from 30-80GB total. I bring at least 96GB to every wedding so that even if one of my cards decides to give up the ghost, I'll never run out of card space.
Also it's very important to pay attention to the speed of your memory cards. When you're shopping for cards you'll come across cards with the same amount of storage for a wide range of prices, and there's a reason. My RAW files are up to 40MB each, and my camera can take 6 of them per second, so up to 250MB of data per second can be flying onto my memory cards. The big bad 1Dx can shoot up to 14 frames per second! The cards of yesteryear were rated to write at around 15mb/sec. If you throw 250MB at them in one second, your camera is going to freeze up and will not take any photos until the buffer empties and all the photos are written to the card. While we don't have memory cards available that will write fast enough to take that much data continuously, we do have cards that write at 150mb/sec, so that's what I use.
Some cards are rated differently, for example 133x 233x 400x 600x or 1000x. In my opinion, 400x (60mb/sec) is now the bare minimum if you're shooting RAW, and if you have a high resolution camera like the 5D MKIII or Nikon D800 you're going to want to go faster to avoid your camera bogging down. While you can buy cards as big as 256GB or still find them as small as 1-2GB, I find that 16GB is a good size because you don't have an entire wedding loaded onto a single card (so if you lose that card you lose the whole wedding) but you don't have to keep up with a dozen different cards if you were using 4GB. I normally bring four 16GB and one 32GB cards. I've gotten flawless performance from my Lexar 1000x cards and I also have Kingston mid-grade cards that I recommend for those who cant' afford the big boys.
When it comes to transferring the images to my computer or laptop, speed is again a factor. When I used to use slow 15mb/sec cards, transferring a full wedding to disc could be a 2hr+ task. Now I use a USB 3.0 card reader, also from Lexar that allows incredibly fast transfer speeds, and I can empty a full 16gb card in less than 2 minutes. There are Firewire card readers that offer similar performance.
Now on to the lenses, the real heart of a good camera system. There's a saying that I love that says, "Amateurs talk about cameras, professionals talk about glass..." (The saying ends with "...Masters talk about light" but that's for another blog post). I talk to up-and-coming photographers on a regular basis and I always advise them that IF they are at a place where they're only going to be investing $500-1000 in upgrading from their "amateur" level camera kit instead of throwing down the full $20,000 for an entire pro level kit, INVEST in GLASS FIRST. Sure, a Rebel or low end Nikon or Sony doesn't have the cajones of a big pro body, but realistically, every camera on the market today is light years ahead of the cameras that true pros used only 10 years ago (minus the aspect of long term durability). Now, 10 years ago it took more talent and experience to get great images out of such relatively limited equipment, but all things being equal, the cameras we have now are pretty amazing.
All those comparisons aside, your camera is only a tool. I used to feel offended when someone would say "Wow, that's a great shot, you must have an awesome camera!" But now I just reply, "Yup! I taught it everything it knows!"
Even though cameras get outdated on a seemingly monthly basis, if you buy good lenses, you can keep them for a very long time with no problems. When I started in photography full time, I made the mistake of buying lenses as fast as I could, finding the cheapest version of a certain type of lens that I could, and I ended up finding the limitations of those lenses very quickly, selling them for a big loss, and spending alot more money on the lenses that I really wanted. For this reason, I also advise photogs looking to move up to do whatever they can to invest in gear that they know they won't have to replace soon. Generally speaking Canon's "L" series lenses are awesome, but there are lenses from other brands that may have a place in your bag as well. I'll list my current lens lineup in order of which spend the most time on my camera.