I'm fairly new to photography too, but here's my advice:
1) The 400d is still a fairly good camera for what it is, ie an entry level DSLR. Learn to use it first before you consider buying a new body. Then, when you are certain you need a new body, decide what you want to photograph and then use this decision to guide you to the right body (ie, if yoiu like wildlife the 7D might be your choice whereas if you like portraits or landscapes a 5DIII or 6D might be better)
2) It is usually better to invest money in new lenses first rather than a new body. However, this doesn't mean you should rush out and immediately buy a Canon L series lens. The exact lens you are best off with will depend entirely on what type of photography you plan to use it for. Play around with your 400d and the 18-55 for a while and get an idea of the type of photography you want to do, then research which lens will be the best to accomplish this. You could consider an EF-S 55-250 zoom to compliment your 18-55 for the time being, if you find you need a bit more reach, they can be bought very cheaply used.
If you go out and buy new kit without really having an idea what you'll be photographing, you'll probably end up regretting your purchase.
3) Used lenses are a good way to go if you have a source you can trust if you want your budget to go a bit further. I use a dslr forum here in Germany to buy used lenses and it has worked out well so far. Decent lenses (e.g. Canon L series) hold their value quite well so if you buy used but then decide the lens isn't for you, you can sell it will little loss. ALWAYS keep the original boxes and invoices for when you want to sell on.