Inline with questions 2 & 3 - what's the bottleneck? reading or writing or both? if you haven't already, it's worth looking into a master (writes) / slave (reads) setup - provides a bit of redundancy and lets you scale reads very easily by adding more slaves. It also helps with writes because it can take a lot of pressure off the master.

...assuming you're webvet.com it looks like you're scaling drupal, so these would be my first questions: are you using the views module? if so, kill that, as well as anything else by merlin (panels, etc). and look at your indexes. setup pressflow + percona. master & slave db. reverse proxy caching (varnish or nginx) are all good places to start. hostwise if you're not already on amazon, you can get a lot of mileage out of a service like voxel that lets you mix VPSs (cheap webheads) with physical hardware. it's not great and it may well be worth biting the more expensive bullet and going to AWS for long term needs, but it's a lot better than linode for an underwater db.

my email is in my profile if you'd like more detail - drupal is a bear at first but it can be scaled for a while.