I have been a Canon "fanboy", as you call it, for over 10 years, starting my path with film Rebel Ti200 and transitioning to digital with Digital Rebel->20D->30D->40D->50D and 5D MKII recently, and owned only high end Canon L lenses such as 70-200mm f2.8 IS USM, 24-105mm, 28-300mm and etc. During all this time I always had that unsatisfied feeling about my shots. With Canons it was always hit or miss for me. Pictures often would come out softer than desired, not a great white balance and exposure performance, almost every picture taken would require some or heavy post processing in Photoshop or Lightroom. So I upgraded camera body with every new model in hopes to get better results, but desired results would just not be there. It tool 5 years! for Canon to introduce spot metering starting with 40D model! And what there to expect from an old 9 point diamond shaped focusing system that was implemented from one model release to the next. Asking myself, after spending so much money on such high end equipment, why do I have to spend so much time on post processing? At that time I didn't even want to hear about Nikon, I despised those Nikon "fanboys". After much hesitation I finally talked myself into trying it. What a shocker it was to me when I compared the images taken under the same conditions from Nikon D60 and Canon 50D with 24-105F4L lens(by the way the best and sharpest Canon lens I owned excluding primes.)! I could not believe how quality and especially focusing and color rendering was better in images from Nikon. I felt devastated. Lost my desire to take pictures for the rest of the trip. Honestly, I felt stupid denying Nikons for so long and not giving it a try sooner.
Came back, made an extensive research on Nikon cameras, sold ALL my Canon equipment worth of around 10k without a blink of hesitation and bought Nikon D700 full framer. Boy, am I enjoying every single shot I take with this camera. Precise focus (51 points with 3D tracking) which Canon is just introduced in their high end 1DMkIV camera (45 points). Sweet enjoyable experience with every shot I take, don't even want to look back at Canon anymore. Now I can actually spend more time shooting than post processing. Yes, comparable Nikon's glass is much more expensive, but it holds the price over the time, which I can't say about Canon's. Every Canon owner that tried my D700 would bring a joyful smile on my face when I see their reaction when the shutter release button is pressed on my Nikon. It's not only the picture quality, but touch and feel of the body along with highly accessible and customizable controls, shutter release sound that make them say WOW! Don't get me wrong Canon produces nice cameras and 5D MkII is not an exception. What I'm saying here is with Canons it takes so much more effort to take great pictures, and with Nikons it comes so natural. So my 5D is already on its way to an eBay auction. No more out of focus shots and focus hunting problems.
So I would say to you GO FOR IT without any hesitation.
The most important component for producing great images is the photographer followed by the lens and then the camera. With good technique and some creativity, you can produce wonderful images with that Rebel or any other entry level camera. These cameras are capable of producing excellent image quality and you might be really surpirsed by what they can do with a high quality lens.
So, my question to you is what do you want the D700 to do for you that the Rebel can't?
David, you should keep an eye on nikonrumors [dot] com
They monitor all of the word on the street. Current consensus is that there is not going to be a refresh for the D700 this year.
I have owned one for a year and a half, and am absolutely thrilled with it. I will keep it for a very long time. The overall sensor performance (low-light, etc.) is still one of the best in the world. see dxomark [dot] com
I think it would be hard to quantify the risk. 2 slots would reduce that risk but I would recommend a different Nikon if you're thinking of buying the D700 new from Amazon. For less that what Amazon sells the D700 for ($3400) you could get a D800 ($2800)or D800E if you want to spend a little more than the D700 costs. The D800/E accommodate 1 CF card and 1 SD card. I'm not a professional but I've taken almost 20,000 shots with my D700 since I bought it new about 1.5 years ago. I use Lexar Compact Flash 1000x UDMA cards (16GB and 32GB) because they're fast. I haven't had any failures yet. In the 12+ years that I've been doing digital photography I've only had failures with Viking cards so I quit buying that brand. I've also used SanDisk, Kingston and Transcend brand cards with no problems.
I don't know what they have it listed for, but for our use, it is worth it's weight in gold. We bought two of them. We take real estate pictures and the newer ones have files way too big to load in quantity.. We bought one on line from an Amazon vendor, but it turned out to be gray market and used even though it was advertised as new. It was from Crawfords. They took it back and we got a better deal from Best Buy. But it isn't made anymore, so you have to pay what you have to pay. If you don't use it professionally like we do, you might be happy with something else.
Depends on amount of light you had. If light is adequate 640 will be clear. I have shot 6400 with lower light for B&W and had normal grainy look. I have shot at 3200 with good light and they were clear