>>I was thinking of selling the remote and getting the 550EX.
If you can afford it, keep the remote. It's a handy piece of kit.
>>Other than the fact that it is an excellent flash, what benefits will I gain?
More light, more control, the ability to use turbo battery packs. use as master or slave, control multiple slave groups, flash ratio control. ETC.
>>Can I use it in full manual mode
Yep. It's manually adjustable from full power to 1/128th. Mind you it has no auto f-stop settings if that is what you mean. Pity too as those are super handy.
>>and control the 420s manually?
Not exactly, but sorta. The best answer is no. It's kinda complicated. You can use flash ratios, but it'll still be E-TTL. If you want to get full manual you really gotta step up to the 550s.
>>Can the zoom be operated manually?
Yup. works like a charm.
Hope this helps a little.