Have you noticed which pages you're getting the 500 errors on?
I've only used PS for the web server, but the answer here depends on where your site is slow. As someone mentioned here in another thread, you might just have to create a few indices on some of your tables.
I personally think PS is worth it to avoid variability in response times on my sites. Before I got PS, my sites were mostly fast enough, but had occasional times where they would slow down and it would take 10s or more to generate a simple WordPress page.
Have you spent much time doing performance analysis and optimizing your site and database? If not, you should think about doing so because you'll definitely be doing so afterwards when every MB and CPU cycle costs money after you've switched to PS!
BTW, the big differentiator of Dreamhost PS is that it is VPS but with the same managed hosting that you get our of a shared hosting account. I have friends who have VPS's and they complain about having to do all the maintenance, updates, and installations "by hand".
What are [color=#CC0000]50DISK50[/color], [color=#CC0000]3DOM50[/color], and [color=#CC0000]1IP1DOM50[/color]?
They're Dreamhost coupons!