Here are my two cents. This site is too slow. Add w3-cache with page level caching and you are good to go.

If you want to move ->
Go with Media Temple GS and get the $20 per month Mysql container. $40 per month and it is very fast. Scales all day, and you can move your DB to faster platform with more money.

Also, use WP tuner to find your long queries and remove them, or use the built in Transient API to store the results. You have to be careful with the Transient API though because if you have a very high traffic site, a short cache time and a long query, you can create a loop/race condition where the cache never gets made right.

What out for stuff like tag clouds, related posts, most comments as they can require a lot of query time and really eat up your server.

I have learned a lot by trial and fire lately about High performance wp latetly moving a site with 5M page views from blogger to WP. WP can handle it, but you have to look at each query and make sure it is necessary. Page level caching with apache redirect (w3-cache enhanced) will take all the load off your server. We are hosting this site at Rackspace cloud sites and using maxCDN and love both. MaxCDN is dead simple to set up with w3-cache.. Total hosting cost about $300 per month to do a TB or so of transfer.

With your traffic and number of comments, you should be fine on Media temple and let it grow with you.

One more comment and I will get off my soap box. Why don't you have a nice big 250 x 300 google image ad on your site (or two) in the sidebar. With 50K page views, you should be able to make about $100 per month. That is enough to pay for some great hosting.