A Canon XSi is not a bridge camera; it is an SLR. A bridge camera is something like the Canon G12, S95, etc. It's an advanced point-and-shoot that has many of the capabilities of an SLR, such as control over aperture and shutter speed, RAW capability, sometimes a flash hotshoe, etc.
I would recommend getting an intro-level SLR, if you're planning on getting serious into photography. Even a basic SLR beats out the image quality and noise-handling of even the better advanved P&S cameras, they track motion, can readily accept upgrades, etc. I recently had the pleasure of helping a friend create a macro setup for his work, and we settled on the D3100, based on my experience with the D40. Since you say that you are a Nikon shooter already, I can wholeheartedly recommend the D3100 as a beginner camera. Use the 18-55 that comes with it until you are completely comfortable with the camera body. After that, any equipment you buy can carry over to your next camera body, when you're ready in however long. I'm seeing it reliably from $600 for the camera with 18-55 (at a shop like
B&H photo), or for $700 with 55-200 VR as well. There are currently instant rebates if you buy lenses at the same time. Buydig's (a.k.a. Beach Camera, an authorized Nikon dealer) was the easiest to find using a quick Google search:
BuyDig.com
So, get the camera first. Get yourself a pair of 8GB SD cards, and you're good to go. A good general upgrade path for new users, unless you know what you want and have special shooting needs is:
1. Camera body + basic lens mid-range zoom (D3100 only comes with 18-55, so learn to use it before you decide to upgrade, the popular choices are 18-200, 16-85, Tamron 17-50, etc)
2. Telephoto lens for that "reach out and touch someone" capability. The 55-200 VR is a good inexpensive lens, but it's slow to focus and not true AF-S (it will autofocus with your camera, but it doesn't have the quick silent focusing with manual override). If you can swing buying the 70-300 VR, you won't be disappointed.
3. A decent tripod and head.
4. A good flash, like the Sb-600 or SB-700.
Feel free to modify the list for your personal preferences. If you do a ton of still-life or landscape, you'll want to get the tripod first. If you do macro, then add a macro lens like the 85mm micro, or the Tamron/Tokina/Sigma to the list. Heck, if you run across another brand camera at like your Costco, go ahead and invest in that. Canon, Pentax, Panasonic/Olympus, etc. all have pretty solid setups. You won't go wrong with any of them. The friend that I helped build the macro setup for uses a Pentax K-5 as his main camera.
If you do go Nikon, then I recommend picking up the 35mm f/1.8 lens as well, since it's such a low price right now, refurbished directly from Nikon:
xhttp://shop.nikonusa.com/store/nikonusa/en_US/pd/productID.213466700?resid=Tk2D3goBAlcAAFGGdgkAAAAD &rests=1313702878140
It will really give you insight into what you can expect, as far as image quality, depth of field, and low-light capability from the more professional lenses.