I recently changed server host with no problems. I put up a copy on the new server and blocked google from the new server using robots.txt, then watched it like a hawk and removed the old pages on the old server and removed the robots.txt block when the new DNS had propagated fully.
This was to avoid duplicate content which is really the only thing to worry about if you're moving servers. All took about 2 days and went very smoothly.
I've got a dedicated IP, mostly because I've had a few issues in the past with other people on my IP spamming and being blacklisted as a result. I don't know how common this problem is though, but it's so cheap I think if you can afford it - go for it.
As for UK or US, I'm currently on UK, but considering moving over to US for some new sites, spose it depends on where the majority of your visitors come from (or where you want them to come from) - has anyone here moved from one to the other and noticed any difference?
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