Welcome to POTN, Roger. It's not a cheeky question at all.
£400 isn't an amazingly good price for a 400D with the kit lens: new ones are currently £479 from Warehouse Express
, but Canon are still running a £50 cashback offer so the net price new would be £429. (And you could get it in black which is more cool.)
The difference between the 6Mp of your Ixus 800 and the 10Mp of the 400D isn't as great as you'd think. It sounds like a big difference but it's only 30% more pixels in each dimension: the images are about 3900x2600 pixels instead of 3000x2000. You won't see much difference, even with an A4-sized enlargement.
Things that will make a difference:
- there's no electronic viewfinder on the 400D, just a traditional optical one
- the kit lens doesn't have as much zoom range as the Ixus 800: in film-camera equivalents its roughly 30-90mm compared with the 35-140mm you're used to
- there's much much less shutter lag on a DSLR
- the low-light performance of a DSLR is vastly superior because the sensor is much bigger
- the larger sensor also means you can take photos with creatively blurred backgrounds
- pictures straight out of the camera are generally less processed, which is good because you have more control but bad if you want to just point & shoot
- you can spend a huge fortune on extra lenses etc. (and the people at POTN will be only too pleased to help you!)
As khall says, it's a whole new ball game. Many people find quite a steep learning curve with DSLRs and only you know whether you want to go for it. If you do, POTN is here to help.