For your needs I would go with the D90 and 2 lenses, a nice prime (I have the 85mm 1.8 which I love) and a zoom lens. You will find, I believe that the D90 is capable of getting what you want, the key is to get good glass. Remember, a camera is only as good as its glass.. the D700, even though it's full frame, would be a lesser camera with 'crap' glass.
As an active professional photographer and photography teacher with more than 40 years of experience I must respectfully disagree with the previous answer. To compare the D90 with the D700 is to compare an amateur camera with a pro camera. The D700, being a full frame camera, will allow shooting at much higher ISOs without significant noise. From your description, available light shooting in low light situations is of prime concern. For that reason alone the D700 would be a better choice. I have two D700s and I have students with D90s so I have experience with both. As for the previous answer's assertion that the D700 has
I am also an experienced professional and I absolutely agree with pawhite. If money is not a big issue, you should absolutely go for the the D700 for two reasons.
1. The image sensor will allow you to shoot in very low light for your interiors without flash, giving you significantly better images at high ISO settings with much less noise.
2. The larger image sensor will allow you to have a wider angle of view using the same focal length lenses. This is a huge factor when wanting to shoot wide angle interiors. This will also allow you to buy lenses that are not as wide which will save you money as well.
I wouldn't even consider the D90 in your situation.
Some good points have been made, but as a working architectural photographer here are the facts. Either camera will produce excellent results. The assumption is that you will shoot at high-ISOs because you're shooting with available light. That is not necessarily true. What is true is that you will need to shoot with a tripod and not handheld. Using your tripod allows you to use lower ISO settings and still retain great resolution and sharpness. Although the technical aspects mentioned above are true, in practical use you won't need to shoot at extremely high ISO settings. The other factor to consider is lens cost. Nikon's non-DX lenses are substantially more expensive than their DX line. Besides, shooting interiors with a lens wider than 21mm (14mm on the D90) does not provide flattering interior perspectives. I guess the bottom line is, if you can afford the D700 that's great, but with the right lens and use of a tripod you can get excellent results from the D90. Take a look at my work. Everything is shot using natural light at ISO 100.
D700 shooter for over a year now... Absolutely the best camera I've ever owned. I shot a racquetball tourney last week... (crappy lighting shot at 3200 ISO/1000th of a sec/1.4 to freeze the action)with reasonable noise (very little after photoshop) and decent dof. Another photog with the D90...reasonable results...but not nearly as good.
I just bought a D700 (previously had a D300) and am falling in love! I see a huge difference in the photos when I use my D700 + SB-900 flash, versus when I used to use the D300 with SB-600 flash.
My shots are coming out a lot nicer, and you can shoot at higher ISO without as much noise. The FX image sensor is definitely something to consider for the D700 over the D90. What I'd recommend though, if you don't have tons of money, is spending it first on your lenses because you can always upgrade your body, but you definitely need some good glass!