I just moved this domain and a few others over to a Dreamhost VPS (Virtual Private Server).
I have WordPress, Gallery 3 and Piwik installed, plus several Joomla sites.
To give you an idea of traffic affecting the VPS this Blog get's between 200 and 300 unique visitors per day. But is also heavily indexed by Google.
Initially I set my VPS's memory usage to the minimum of 300MB to save on costs. But then I started getting server not available when clicking the Publish button in WordPress and a visitor commented on seeing server 500 errors.
After lodging a support request with Dreamhost I discovered that I was hitting a memory ceiling
psXXXXX exceeded memory ceiling, killed all processes in context -
2011-08-12 18:49
psXXXXX exceeded memory ceiling, killed all processes in context -
2011-08-14 01:54
psXXXXX exceeded memory ceiling, killed all processes in context -
2011-08-14 02:05
psXXXXX exceeded memory ceiling, killed all processes in context -
2011-08-14 02:30
psXXXXX exceeded memory ceiling, killed all processes in context -
2011-08-14 02:40
To rectify the situation they recommend a whole range of options from setting stricter robots.txt rules (to stop Google and others consuming so much ram with high levels of spidering) to disabling wordpress plugins that are resource hungry.
I have chosen to increase my RAM to 400MB, which has an accompanying financial penalty sigh.
I notice on the VPS there doesn't seem to be any swap file configured so there isn't any way for it to make up for a lack of physical RAM by paging to disk. So you need to purchase as much RAM as you will actually use.
Anyway just a warning, if you think that you can get away with just the minimum RAM allowance you may find that your previously slow sites on shared hosting become frequently unavailable on a VPS due to hitting a hard limit.
I have been using hostgator for VPS hosting and have had none of the issues you describe. Maybe worth a look.