Just got a 450D, have some questions and need a FD adapter.

You don't want an FD adapter. It's physically impossible to properly mount an FD lens on an EOS camera, because the FD lens is designed to be mounted 42 mm from the film, while the EOS mount sticks out 44 mm from the film/sensor. You'd need an adapter that's -2 mm thick. Unless you're comfortable working in the fifth dimension, that's not going to happen.

There are two kinds of adapters out there for FD-to-EOS. Ones without glass are essentially macro extension tubes; the lens is unable to focus at infinity or, in fact, beyond a few feet. Ones with glass suck; you could save money by buying a $25 Holga lens for your camera. There's one exception, but it's not relevant: Canon once made a very nice adapter for some specific telephoto FD lenses, mostly L lenses, but you couldn't afford it even if you had the appropriate lenses.

Also, take into account that any adapter with glass will increase the focal length by around 1.25x, and thereby reduce the effective aperture. Your 50mm f/1.4 would become about 65mm f/1.8, and then you have to deal with the Field-of-View crop from the smaller sensor on the XSi/450D. You might as well buy the Canon EF 50mm f/1.8 II lens, which goes for about $100 brand new.

Some of the new "mirrorless" cameras do have usable FD adapters available, because their lenses are designed to be mounted much closer to the sensor. There's still the "crop factor" to be dealt with, though.

And yes, of course the lens would be manual-focus only. On the XSi/450D, the viewfinder screen shows almost everything as being in focus whether it is or not, so that's a problem. Some of the "mirrorless" cameras have better manual-focus displays, such as 10x-zoom and focus-peaking.