Hi Oli, welcome to the Cameralabs forums!
1: The kit lens is certainly the weak point of the Canon 400D / XTi, but it's not all bad - I've taken plenty of good photos with it. But at the same time, if you can afford it, it's a great idea to upgrade it.
The thing to remember though is most DSLRs have smaller sensors than a frame of 35mm film, so any lenses you attach effectively have their focal lengths multiplied. In the case of the Canon 400D, they're all multiplied by 1.6 times, so the 28-135mm would act like a 45-216mm lens.
This might mean it has a more powerful telephoto, but it also means it no longer has wide angle capabilities. In order to deliver the same wide angle cover age as a 28mm on a film camera, you'd need 18mm on most DSLRs - that's why most DSLR kit lenses have 18-55mm focal lengths.
So unless you're happy losing wide angle capabilities, I'd avoid the 28-135mm lens and upgrade to something like the Canon EF-S 17-85mm. See our full review at http://www.cameralabs.com/reviews/Canon1785EFS/
. You may also want to consider the Sigma 18-200mm, again reviewed at http://www.cameralabs.com/reviews/Sigma18200mm/
. Or of course stick with the kit lens, at least for the time being! It doesn't add much to the price and gives it better resale value.
2: The Olympus E-510 looks like being a very impressive camera, and so long as the new sensor isn't noisy, I think it could easily become one of the best DSLRs around. The twin lens kit should also cover you for most types of shots. So if you can wait until June / July, I would advise seeing how it works out in practice. We will of course be reviewing it at Cameralabs, and there's a full preview here:
http://www.cameralabs.com/analysis/Olym ... ympusE510/