Also, does anyone know if running a uk website on a US server has a negative impact on SEO?
There are two ways Google identifies websites with different countries: 1)by domain name, 2)by IP location.
If you use ".uk" domain it does not matter that the server is located in US, as google will show your site in the UK results. But if you use a ".com" domain, it is better you host your site in the UK if you want it to appear in the UK search results.
If you have a ".uk" domain I would definetely go for a US webhosting company, as they offer much better deals for cheaper prices.
How many website would you run on a server like this?
That depends how much traffic each website gets, and also how resource hungry those websites are. It is not the same a database driven website, than a plain HTML website. The first one needs a lot more resources than the second.
All the processors do the same, but the difference between them is that some are more powerful than others.
There are many web hosting companies you can look: softlayer, layeredtech, servermatrix, hivelocity, gnax, and many others.