I have owned the 400d for around 3 years now. It is a good camera in places. If you want to get into Infra-Red pictures, I would suggest buying a cheap R72 filter. I say cheap, because the 400d is not great for IR I think. There are two reasons for this, the camera has an internal IR filter, which blocks the infra-red light coming in. The camera does not handle noise on long exposures, or noise from high ISO that well.
The latest version of the firmware, 1.1.1, made a large difference to my camera and the handling of the noise. Uploading the firmware is easy enough.
The kit lens has a 58mm filter, you can also get stepping rings, that change the filter size, so if you are feeling flush, it is an idea to get the largest size you are likely to need, and use stepping rings to get there. For me, the largest size I have is 77mm. For filters which increase the time of an exposure a lot (like an R72 IR filter, or an ND filter), there is a school of thought which suggests that screw in filters are best, as they are less likely to allow light to get in the side. If you do do long exposures, remember to use the eyepiece cap as well (should be on the neck strap).
There are two schools on flashguns for basic use. One is to go the cheap and cheerful route, like the ones in Jessops, then you have not had to spend out much if it doesn't work out. However, the cheap route might also mean that the flash requires more manual input.
Or, to go the full-out route, I went this way, with the 580 exII recently. This flash works in an automatic way, it can calculate the distance to the subject, and modify the power output of the flash accordingly. If you want, as a basic user, all you need to do is dial in an amount of exposure compensation should the flash get it slightly wrong. However, if you want to do more interesting things too, it supports multi-flash/strobe modes and full manual control as well.
On the lens front, the 18-55 lens is meant to have been good for a kit lens at the time that it came out. I classify lenses myself as not good, ok, good, great, with unfortunately the price of 'great' ones always being more than the others. On a crop camera, my understanding is that lens focal lengths up to ~50mm are usual for portraits (but rules are there to be broken). There are two things that are kinda important for a portrait, the framing of the person/body part in the picture, and the background.
Most people think that a nice blurred background helps to bring attention to the person in the portrait. For this, you need to have a wide aperture lens. Unfortunately, the wider the aperture, the more expensive the lens. However, there is a good, cheap lens which is a place to start with portraits, and that is the 'nifty fifty', or to give it its real name, the Canon 50mm f/1.8. This is a relatively cheap ~£75 at the moment lens, about the same price of a good filter, and allows one to experiment with large apertures, without a huge outlay.
Should you feel that a fixed focal length is to restrictive, then the Canon 28-135 IS lens is a good lens, I also use mine for some landscapes. The lens has image stabilisation built in. You need to remember though that IS only helps the camera stay still, if there is movement in the picture, it will still be blurred. Although IS can be of benefit to lower focal length lenses, it really comes into its realm of use for long focal lengths. Also on this lens, it has the older type of IS, so you need to remember to turn it off if using it on a tripod (good practice anyway), and does not really support panning a moving target.
