Canon vs Nikon
If you're interested in bird photography, 50D (or 40D / 30D) + 400mm f/5.6L is the cheapest and longest reaching option. Nikon has no cheap alternative for long lenses (not that I know of).
For macro, Nikon has a 105mm VR (vibration reduction) lens that allows to better handholding in less-than-ideal light for macro. However, Canon has a 100mm lens without IS (image stabilisation) that's cheaper. Macro should be done with tripod when and if possible, so a Benro / Yong Nuo tripod would be alright for you (as a beginner, unless you have deep pockets).
For everything else you're going to shoot (family, events etc), both cameras (D90 / 50D) are good and both have good lenses. The 50D might be better on the ISO side, but its subjective.
Canon has been dominating the high-iso low-noise sensor department for a while now (till the Nikon D3 / D300 came out). Their long lenses (perfect for birding and sports) have been the choice of many professionals for quite some time (Pre-2008 olympics, most of the lenses were white, which means they're Canon). Being the #1 brand for professionals for a while (1D2 and 1Ds2 was unbeatable for a while), Canon definitely seems to support their professional buyers more than the average hobbyist, with the Canon Professional Services (CPS) schemes and their awesome top-grade bodies, though their lower-end "perfect-for-hobbyists" models like the 450D and 50D do seem to be a good entry-point for newbies.
Nikon, on the other hand, took the spotlight with their D3 and D300 models when Canon made a big blunder on their top-grade 1D mark3 autofocus. However, for the entry-level cameras, they really toned down the features. Nikon D40/D40x/D60 don't have AF modules in their cameras, for example. Other than the D3 / D700, the high-ISO pictures on every Nikon camera can't really match Canon's 40D / 50D, though the D300 and D90 sure are catching up very quickly now.
For full-frame cameras, D3 has 12mp and 11fps full-frame shooting, while Canon's 1Ds mk3 has 21mp and 5fps shooting. To me (im probably wrong) Canon's full-frame seems to stress more on image quality than speed.
Cheers,
Zexun